xfs_db: use iocursor type to guess btree geometry if bad magic

xfs_db: use iocursor type to guess btree geometry if bad magic

The function block_to_bt plays an integral role in determining the btree
geometry of a block that we want to manipulate with the debugger.
Normally we use the block magic to find the geometry profile, but if the
magic is bad we'll never find it and return NULL.  The callers of this
function do not check for NULL and crash.

Therefore, if we can't find a geometry profile matching the magic
number, use the iocursor type to guess the profile and scowl about that
to stdout.  This makes it so that even with a corrupt magic we can try
to print the fields instead of crashing the debugger.

[sandeen: comment changes, add magic ASSERT, keep other ASSERTs]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>