netlabel: check for IPV4MASK in addrinfo_get

netlabel: check for IPV4MASK in addrinfo_get

netlbl_unlabel_addrinfo_get() assumes that if it finds the
NLBL_UNLABEL_A_IPV4ADDR attribute, it must also have the
NLBL_UNLABEL_A_IPV4MASK attribute as well. However, this is
not necessarily the case as the current checks in
netlbl_unlabel_staticadd() and friends are not sufficent to
enforce this.

If passed a netlink message with NLBL_UNLABEL_A_IPV4ADDR,
NLBL_UNLABEL_A_IPV6ADDR, and NLBL_UNLABEL_A_IPV6MASK attributes,
these functions will all call netlbl_unlabel_addrinfo_get() which
will then attempt dereference NULL when fetching the non-existent
NLBL_UNLABEL_A_IPV4MASK attribute:

Unable to handle kernel NULL pointer dereference at virtual address 0
Process unlab (pid: 31762, stack limit = 0xffffff80502d8000)
Call trace:
	netlbl_unlabel_addrinfo_get+0x44/0xd8
	netlbl_unlabel_staticremovedef+0x98/0xe0
	genl_rcv_msg+0x354/0x388
	netlink_rcv_skb+0xac/0x118
	genl_rcv+0x34/0x48
	netlink_unicast+0x158/0x1f0
	netlink_sendmsg+0x32c/0x338
	sock_sendmsg+0x44/0x60
	___sys_sendmsg+0x1d0/0x2a8
	__sys_sendmsg+0x64/0xb4
	SyS_sendmsg+0x34/0x4c
	el0_svc_naked+0x34/0x38
Code: 51001149 7100113f 540000a0 f9401508 (79400108)
---[ end trace f6438a488e737143 ]---
Kernel panic - not syncing: Fatal exception

Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>