xfsprogs: metadump: use printable characters for obfuscated names

xfsprogs: metadump: use printable characters for obfuscated names

There is probably not much need for an extreme amount of randomness
in the obfuscated names produced in metadumps.  Limit the character
set used for (most of) these names to printable characters rather
than every permittable byte.  The result makes metadumps a bit more
natural to work with.

I chose the set of all upper- and lower-case letters, digits, and
the dash and underscore for the alphabet.  It could easily be
expanded to include others (or reduced for that matter).

This change also avoids ever having to retry after picking an
unusable character.

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