netlink, mmap: don't walk rx ring on poll if receive queue non-empty

In case of netlink mmap, there can be situations where received frames
have to be placed into the normal receive queue. The ring buffer indicates
this through NL_MMAP_STATUS_COPY, so the user is asked to pick them up
via recvmsg(2) syscall, and to put the slot back to NL_MMAP_STATUS_UNUSED.

Commit 0ef70770 ("netlink: rx mmap: fix POLLIN condition") changed
polling, so that we walk in the worst case the whole ring through the
new netlink_has_valid_frame(), for example, when the ring would have no
NL_MMAP_STATUS_VALID, but at least one NL_MMAP_STATUS_COPY frame.

Since we do a datagram_poll() already earlier to pick up a mask that could
possibly contain POLLIN | POLLRDNORM already (due to NL_MMAP_STATUS_COPY),
we can skip checking the rx ring entirely.

In case the kernel is compiled with !CONFIG_NETLINK_MMAP, then all this is
irrelevant anyway as netlink_poll() is just defined as datagram_poll().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>