xfs: simplify __xfs_buf_ioend

xfs: simplify __xfs_buf_ioend

__xfs_buf_ioend can only resubmit the buffer for asynchronous
writes, which means the retry handling xfs_buf_iowait is not needed.

Because of this can stop returning a value from __xfs_buf_ioend and
just release the buffer for async I/O that does not require retries.

Also drop the __-prefix now that the semantics are straight forward.

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