xfs: remove xfs_buf_delwri_submit_buffers

xfs_buf_delwri_submit_buffers has two callers for synchronous and
asynchronous writes that share very little logic.  Split out a helper for
the shared per-buffer loop and otherwise open code the submission in the
two callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>