nf_tables: fix NULL pointer dereference in nft_inner_init()

We should check whether the NFTA_INNER_NUM netlink attribute is present
before accessing it, otherwise a null pointer deference error will occur.

Call Trace:
dump_stack_lvl+0x4f/0x90
print_report+0x3f0/0x620
kasan_report+0xcd/0x110
__asan_load4+0x84/0xa0
nft_inner_init+0x128/0x2e0
nf_tables_newrule+0x813/0x1230
nfnetlink_rcv_batch+0xec3/0x1170
nfnetlink_rcv+0x1e4/0x220
netlink_unicast+0x34e/0x4b0
netlink_sendmsg+0x45c/0x7e0
__sys_sendto+0x355/0x370
__x64_sys_sendto+0x84/0xa0
do_syscall_64+0x3f/0x90
entry_SYSCALL_64_after_hwframe+0x6e/0xd8

Fixes: 3a07327d10a0 ("netfilter: nft_inner: support for inner tunnel header matching")
Signed-off-by: Xingyuan Mo <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>

net/netfilter/nft_inner.c

···
 	int err;
 
 	if (!tb[NFTA_INNER_FLAGS] ||
+	    !tb[NFTA_INNER_NUM] ||
 	    !tb[NFTA_INNER_HDRSIZE] ||
 	    !tb[NFTA_INNER_TYPE] ||
 	    !tb[NFTA_INNER_EXPR])