net: devmem: support single IOV with sendmsg

sendmsg() with a single iov becomes ITER_UBUF, sendmsg() with multiple
iovs becomes ITER_IOVEC. iter_iov_len does not return correct
value for UBUF, so teach to treat UBUF differently.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Mina Almasry <almasrymina@google.com>
Fixes: bd618489 ("net: devmem: Implement TX path")
Signed-off-by: Stanislav Fomichev <stfomichev@gmail.com>
Acked-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>