ipv6: fix static key imbalance in fl_create()

fl_create() should call static_branch_deferred_inc() only in
case of success.

Also we should not call fl_free() in error path, as this could
cause a static key imbalance.

jump label: negative count!
WARNING: CPU: 0 PID: 15907 at kernel/jump_label.c:221 static_key_slow_try_dec kernel/jump_label.c:221 [inline]
WARNING: CPU: 0 PID: 15907 at kernel/jump_label.c:221 static_key_slow_try_dec+0x1ab/0x1d0 kernel/jump_label.c:206
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 15907 Comm: syz-executor.2 Not tainted 5.2.0-rc6+ #62
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 panic+0x2cb/0x744 kernel/panic.c:219
 __warn.cold+0x20/0x4d kernel/panic.c:576
 report_bug+0x263/0x2b0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:986
RIP: 0010:static_key_slow_try_dec kernel/jump_label.c:221 [inline]
RIP: 0010:static_key_slow_try_dec+0x1ab/0x1d0 kernel/jump_label.c:206
Code: c0 e8 e9 3e e5 ff 83 fb 01 0f 85 32 ff ff ff e8 5b 3d e5 ff 45 31 ff eb a0 e8 51 3d e5 ff 48 c7 c7 40 99 92 87 e8 13 75 b7 ff <0f> 0b eb 8b 4c 89 e7 e8 a9 c0 1e 00 e9 de fe ff ff e8 bf 6d b7 ff
RSP: 0018:ffff88805f9c7450 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 0000000000000000
RDX: 000000000000e3e1 RSI: ffffffff815adb06 RDI: ffffed100bf38e7c
RBP: ffff88805f9c74e0 R08: ffff88806acf0700 R09: ffffed1015d060a9
R10: ffffed1015d060a8 R11: ffff8880ae830547 R12: ffffffff89832ce0
R13: ffff88805f9c74b8 R14: 1ffff1100bf38e8b R15: 00000000ffffff01
 __static_key_slow_dec_deferred+0x65/0x110 kernel/jump_label.c:272
 fl_free+0xa9/0xe0 net/ipv6/ip6_flowlabel.c:121
 fl_create+0x6af/0x9f0 net/ipv6/ip6_flowlabel.c:457
 ipv6_flowlabel_opt+0x80e/0x2730 net/ipv6/ip6_flowlabel.c:624
 do_ipv6_setsockopt.isra.0+0x2119/0x4100 net/ipv6/ipv6_sockglue.c:825
 ipv6_setsockopt+0xf6/0x170 net/ipv6/ipv6_sockglue.c:944
 tcp_setsockopt net/ipv4/tcp.c:3131 [inline]
 tcp_setsockopt+0x8f/0xe0 net/ipv4/tcp.c:3125
 sock_common_setsockopt+0x94/0xd0 net/core/sock.c:3130
 __sys_setsockopt+0x253/0x4b0 net/socket.c:2080
 __do_sys_setsockopt net/socket.c:2096 [inline]
 __se_sys_setsockopt net/socket.c:2093 [inline]
 __x64_sys_setsockopt+0xbe/0x150 net/socket.c:2093
 do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4597c9
Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f2670556c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 00000000004597c9
RDX: 0000000000000020 RSI: 0000000000000029 RDI: 0000000000000003
RBP: 000000000075bfc8 R08: 000000000000fdf7 R09: 0000000000000000
R10: 0000000020000000 R11: 0000000000000246 R12: 00007f26705576d4
R13: 00000000004cec00 R14: 00000000004dd520 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..

Fixes: 59c820b2 ("ipv6: elide flowlabel check if no exclusive leases exist")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>