nlmon: fix comparison in nlmon_is_valid_mtu

This patch fixes the following warning introduced in e4fc408e
("packet: nlmon: virtual netlink monitoring device for packet
sockets") reported by Dan Carpenter:

warning: "drivers/net/nlmon.c:31 nlmon_is_valid_mtu()
	 warn: always true condition '(new_mtu <= ((~0 >> 1))) =>
				      (s32min-s32max <= s32max)'"

Thus, we should simply remove the test against INT_MAX. Next to that
we also need to explicitly cast the sizeof() case as the comparison
is type promoted to unsigned long so negative values are then
valid instead of invalid. While at it, this also adds a comment about
Netlink and MTUs.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>