xfsprogs: metadump: fix duplicate handling once and for all

xfsprogs: metadump: fix duplicate handling once and for all

This is a case where I think I've solved a problem to death.

The metadump code now stops rather than spinning forever in the face
of finding no obfuscated name that hasn't already been seen.
Instead, it simply gives up and passes the original name back to use
without obfuscation.

Unfortunately, as a result it actually creates entries with
duplicate names in a directory (or inode attribute fork).  And at
least in the case of directories, xfs_mdrestore(8) will populate the
directory it restores with duplicate entries.  That even seems to
work, but xfs_repair(8) does identify this as a problem and fixes it
(by moving duplicates to "lost+found").

This might have been OK, given that it was a rare occurence.  But
it's possible, with short (5-character) names, for the obfuscation
algorithm to come up with only a single possible alternate name,
and I felt that was just not acceptable.

This patch fixes all that by creating a way to generate alternate
names directly from existing names by carefully flipping pairs of
bits in the characters making up the name.

The first change is that a name is only ever obfuscated once.
If the obfuscated name can't be used, an alternate is computed
based on that name rather than re-starting the obfuscation
process.  (Names shorter than 5 characters are still not
obfuscated.)

Second, once a name is selected for use (obfuscated or not), it is
checked for duplicates.  The name table is consulted to see if it
has already been seen, and if it has, an alternate for that name is
created (a different name of the same length that has the same hash
value).  That name is checked in the name table, and if it too is
already there the process repeats until an unused one is found.

Third, alternates are generated methodically rather than by
repeatedly trying to come up with new random names.  A sequence
number uniquely defines a particular alternate name, given an
existing name.  (Note that some of those alternates aren't valid
because they contain at least one unallowed character.)

Finally, because all names are now maintained in the name table,
and because of the way alternates are generated, it's actually
possible for short names to get modified in order to avoid
duplicates.

The algorithm for doing all of this is pretty well explained in
the comments in the code itself, so I'll avoid duplicating any
more of that here.

Updates since last posting:
    - Definition of ARRAY_SIZE() macro moved to "include/libxfs.h"
    - Added some more background commentary:
	- About the details of operation in flip_bit().
	  Specifically, that the table can be expanded as needed,
	  but that it is already way bigger than practically
	  necessary (and why it is that way).
	- About the number of alternates available as the length
	  of a name increases.
	- That the key cases we're interested in are names that are
	  around 5 characters in length.  Less than that it's not
	  very important because we don't obfuscate the name, and
	  greater than that the odds of the result of conflicting
	  with an existing name are small.
    - Basically, the density of meaning in this code is kind of
      high, so it warrants a lot more comments to help make what
      it's doing more apparent.  So I fleshed this out, as requested
      by Dave.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>