xfs: store an ops pointer in struct xfs_defer_pending

The dfp_type field in struct xfs_defer_pending is only used to either
look up the operations associated with the pending word or in trace
points.  Replace it with a direct pointer to the operations vector,
and store a pretty name in the vector for tracing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>