xfs_repair: validate alignment of inherited rt extent hints

xfs_repair: validate alignment of inherited rt extent hints

If we encounter a directory that has been configured to pass on an
extent size hint to a new realtime file and the hint isn't an integer
multiple of the rt extent size, we should turn off the hint because that
is a misconfiguration.  Old kernels didn't check for this when copying
attributes into new files and would crash in the rt allocator.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[sandeen: clarify that it is an extent size *hint*]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>