The following warning is triggered every time an unestablished mesh peer
gets dumped. Checks if a peer link is established before retrieving the
airtime link metric.

[ 9563.022567] WARNING: CPU: 0 PID: 6287 at net/mac80211/mesh_hwmp.c:345
               airtime_link_metric_get+0xa2/0xb0 [mac80211]
[ 9563.022697] Hardware name: PC Engines apu2/apu2, BIOS v4.10.0.3
[ 9563.022756] RIP: 0010:airtime_link_metric_get+0xa2/0xb0 [mac80211]
[ 9563.022838] Call Trace:
[ 9563.022897]  sta_set_sinfo+0x936/0xa10 [mac80211]
[ 9563.022964]  ieee80211_dump_station+0x6d/0x90 [mac80211]
[ 9563.023062]  nl80211_dump_station+0x154/0x2a0 [cfg80211]
[ 9563.023120]  netlink_dump+0x17b/0x370
[ 9563.023130]  netlink_recvmsg+0x2a4/0x480
[ 9563.023140]  ____sys_recvmsg+0xa6/0x160
[ 9563.023154]  ___sys_recvmsg+0x93/0xe0
[ 9563.023169]  __sys_recvmsg+0x7e/0xd0
[ 9563.023210]  do_syscall_64+0x4e/0x140
[ 9563.023217]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20191203180644.70653-1-markus.theil@tu-ilmenau.de
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>