mac80211: minor cleanup to mesh state locking

First time I tried smatch, and it says:
mesh_hwmp.c +870 mesh_queue_preq(21) error: double lock 'bottom_half:'
mesh_hwmp.c +873 mesh_queue_preq(24) error: double unlock 'bottom_half:'
mesh_hwmp.c +886 mesh_queue_preq(37) error: double unlock 'bottom_half:'

Which is indeed true -- there's no point in disabling BHs
again if we just did that a few lines earlier, so remove.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>