xfs_repair: don't zero the incore secondary super when zeroing

xfs_repair: don't zero the incore secondary super when zeroing

If secondary_sb_whack detects nonzero bytes beyond the end of the ondisk
superblock, it will try to zero the end of the ondisk buffer as well as
the incore superblock prior to scan_ag using that incore super to
rewrite the ondisk super.

However, the metadata directory feature adds a sb_metadirino field to
the incore super.  On disk, this is stored in the same slot as
sb_rbmino, but we wanted to cache both inumbers incore to minimize the
churn.  Therefore, it is now only safe to zero the "end" of an xfs_dsb
buffer, and never an xfs_sb object.

Most of the XFS codebase moved off that second behavior long ago, with
the exception of this one part of repair.  The zeroing probably ought to
be turned into explicit logic to zero fields that weren't defined with
the featureset encoded in the primary superblock, but for now we'll
resort to always resetting the values from the xfs_mount's xfs_sb.

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