mac80211: fix smatch lock errors in mesh

smatch was complaining:

CHECK   net/mac80211/mesh_pathtbl.c
net/mac80211/mesh_pathtbl.c:562 mesh_path_add() error: double lock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:580 mesh_path_add() error: double unlock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:589 mesh_path_add() error: double unlock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:691 mpp_path_add() error: double lock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:707 mpp_path_add() error: double unlock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:716 mpp_path_add() error: double unlock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:814 mesh_path_flush_by_nexthop() error:
double lock 'bottom_half:'
net/mac80211/mesh_pathtbl.c:819 mesh_path_flush_by_nexthop() error:
double unlock 'bottom_half:'
net/mac80211/mesh_pathtbl.c:887 mesh_path_del() error: double lock
'bottom_half:'
net/mac80211/mesh_pathtbl.c:901 mesh_path_del() error: double unlock
'bottom_half:'

So don't lock / unlock with _bh() while bottom halves are already
disabled.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>