xfs: always complete the buffer inline in xfs_buf_submit

xfs_buf_submit now only completes a buffer on error, or for in-memory
buftargs.  There is no point in using a workqueue for the latter as
the completion will just wake up the caller.  Optimize this case by
avoiding the workqueue roundtrip.

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>