xfs: explicitly initialize meta_scrub_ops array by type

An implicit mapping to type by order of initialization seems
error-prone, and doesn't lend itself to cscope-ing.

Also add sanity checks about size of array vs. max types,
and a defensive check that ->scrub exists before using it.

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