xfs: remove all boilerplate defer init/finish code

At this point, the transaction subsystem completely manages deferred
items internally such that the common and boilerplate
xfs_trans_alloc() -> xfs_defer_init() -> xfs_defer_finish() ->
xfs_trans_commit() sequence can be replaced with a simple
transaction allocation and commit.

Remove all such boilerplate deferred ops code. In doing so, we
change each case over to use the dfops in the transaction and
specifically eliminate:

- The on-stack dfops and associated xfs_defer_init() call, as the
  internal dfops is initialized on transaction allocation.
- xfs_bmap_finish() calls that precede a final xfs_trans_commit() of
  a transaction.
- xfs_defer_cancel() calls in error handlers that precede a
  transaction cancel.

The only deferred ops calls that remain are those that are
non-deterministic with respect to the final commit of the associated
transaction or are open-coded due to special handling.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>