packet: Avoid lock_sock in mmap handler

As the mmap handler gets called under mmap_sem, and we may grab
mmap_sem elsewhere under the socket lock to access user data, we
should avoid grabbing the socket lock in the mmap handler.

Since the only thing we care about in the mmap handler is for
pg_vec* to be invariant, i.e., to exclude packet_set_ring, we
can achieve this by simply using a new mutex.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Martin MOKREJŠ <mmokrejs@ribosome.natur.cuni.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>