xfs: create a new buf_ops pointer to verify structure metadata

xfs: create a new buf_ops pointer to verify structure metadata

Source kernel commit: b55725974c9d3a5afcdf83daff6fba7d3f91ffca

Expose all metadata structure buffer verifier functions via buf_ops.
These will be used by the online scrub mechanism to look for problems
with buffers that are already sitting around in memory.

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