libxfs: make xfs_buf_delwri_submit actually do something

libxfs: make xfs_buf_delwri_submit actually do something

xfs_buf_delwri_queue doesn't report errors, which means that if the
buffer write fails we have no way of knowing that something bad
happened.  In the kernel we queue and then submit buffers, and the
submit call communicates errors to callers.  Do the same here since
we're going to start using the AG header initialization functions, which
use delwri_{queue,submit} heavily.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>