nl80211: use netlink consistent dump feature for BSS dumps

Use the new consistent dump feature from (generic) netlink
to advertise when dumps are incomplete.

Readers may note that this does not initialize the
rdev->bss_generation counter to a non-zero value. This is
still OK since the value is modified only under spinlock
when the list is modified. Since the dump code holds the
spinlock, the value will either be > 0 already, or the
list will still be empty in which case a consistent dump
will actually be made (and be empty).

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