xfs: fix buffer state management in xrep_findroot_block

We don't handle buffer state properly in online repair's findroot
routine.  If a buffer already has b_ops set, we don't ever want to touch
that, and we don't want to call the read verifiers on a buffer that
could be dirty (CRCs are only recomputed during log checkpoints).

Therefore, be more careful about what we do with a buffer -- if someone
else already attached ops that are not the ones for this btree type,
just ignore the buffer.  We only attach our btree type's buf ops if it
matches the magic/uuid and structure checks.

We also modify xfs_buf_read_map to allow callers to set buffer ops on a
DONE buffer with NULL ops so that repair doesn't leave behind buffers
which won't have buffers attached to them.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>