xfs: undefine the sb_bad_features2 when metadir is enabled

xfs: undefine the sb_bad_features2 when metadir is enabled

The metadir feature is a good opportunity to break from the past where
we had to do this strange dance with sb_bad_features2 due to a past bug
where the superblock was not kept aligned to a multiple of 8 bytes.

Therefore, stop this pretense when metadir is enabled.  We'll just set
the incore and ondisk fields to zero, thereby freeing up 4 bytes of
space in the superblock.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>