net/sched: act_pedit: fix WARN() in the traffic path

when configuring act_pedit rules, the number of keys is validated only on
addition of a new entry. This is not sufficient to avoid hitting a WARN()
in the traffic path: for example, it is possible to replace a valid entry
with a new one having 0 extended keys, thus causing splats in dmesg like:

 pedit BUG: index 42
 WARNING: CPU: 2 PID: 4054 at net/sched/act_pedit.c:410 tcf_pedit_act+0xc84/0x1200 [act_pedit]
 [...]
 RIP: 0010:tcf_pedit_act+0xc84/0x1200 [act_pedit]
 Code: 89 fa 48 c1 ea 03 0f b6 04 02 84 c0 74 08 3c 03 0f 8e ac 00 00 00 48 8b 44 24 10 48 c7 c7 a0 c4 e4 c0 8b 70 18 e8 1c 30 95 ea <0f> 0b e9 a0 fa ff ff e8 00 03 f5 ea e9 14 f4 ff ff 48 89 58 40 e9
 RSP: 0018:ffff888077c9f320 EFLAGS: 00010286
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffffac2983a2
 RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888053927bec
 RBP: dffffc0000000000 R08: ffffed100a726209 R09: ffffed100a726209
 R10: 0000000000000001 R11: ffffed100a726208 R12: ffff88804beea780
 R13: ffff888079a77400 R14: ffff88804beea780 R15: ffff888027ab2000
 FS:  00007fdeec9bd740(0000) GS:ffff888053900000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007ffdb3dfd000 CR3: 000000004adb4006 CR4: 00000000001606e0
 Call Trace:
  tcf_action_exec+0x105/0x3f0
  tcf_classify+0xf2/0x410
  __dev_queue_xmit+0xcbf/0x2ae0
  ip_finish_output2+0x711/0x1fb0
  ip_output+0x1bf/0x4b0
  ip_send_skb+0x37/0xa0
  raw_sendmsg+0x180c/0x2430
  sock_sendmsg+0xdb/0x110
  __sys_sendto+0x257/0x2b0
  __x64_sys_sendto+0xdd/0x1b0
  do_syscall_64+0xa5/0x4e0
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
 RIP: 0033:0x7fdeeb72e993
 Code: 48 8b 0d e0 74 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d 0d d6 2c 00 00 75 13 49 89 ca b8 2c 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 4b cc 00 00 48 89 04 24
 RSP: 002b:00007ffdb3de8a18 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
 RAX: ffffffffffffffda RBX: 000055c81972b700 RCX: 00007fdeeb72e993
 RDX: 0000000000000040 RSI: 000055c81972b700 RDI: 0000000000000003
 RBP: 00007ffdb3dea130 R08: 000055c819728510 R09: 0000000000000010
 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000040
 R13: 000055c81972b6c0 R14: 000055c81972969c R15: 0000000000000080

Fix this moving the check on 'nkeys' earlier in tcf_pedit_init(), so that
attempts to install rules having 0 keys are always rejected with -EINVAL.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>