The wmb() in next_desc seems out of place and the comments are
inaccurate. Remove the unnecessary barrier and clean up next_desc().

next_desc() is called by virt_queue__get_head_iov() when filling the iov
with desciptor addresses. It reads the descriptor's flag and next index.
The virt_queue__get_head_iov() only reads the direct and indirect
descriptors, and doesn't write any shared memory except from iov and
cursors that will be read by the caller.

As far as I can see, vhost (the kernel implementation of virtio device)
does well without any barrier here, so I think it might be safe to remove.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>