xfs: simplify xfs_buf_delwri_pushbuf

xfs_buf_delwri_pushbuf synchronously writes a buffer that is on a delwri
list already.  Instead of doing a complicated dance with the delwri
and wait list, just leave them alone and open code the actual buffer
write.

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>