xfs: fix handling of synchronous errors in xfs_buf_submit

xfs: fix handling of synchronous errors in xfs_buf_submit

Synchronous readers and writers already run __xfs_buf_ioend from
xfs_buf_iowait after being woken through bp->b_iowait, so we
should not call it here, which can lead to double completions.

Fixes: 4b90de5bc0f5 ("xfs: reduce context switches for synchronous buffered I/O")
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>