wext: let get_wireless_stats() sleep

A number of drivers (recently including cfg80211-based ones)
assume that all wireless handlers, including statistics, can
sleep and they often also implicitly assume that the rtnl is
held around their invocation. This is almost always true now
except when reading from sysfs:

  BUG: sleeping function called from invalid context at kernel/mutex.c:280
  in_atomic(): 1, irqs_disabled(): 0, pid: 10450, name: head
  2 locks held by head/10450:
   #0:  (&buffer->mutex){+.+.+.}, at: [<c10ceb99>] sysfs_read_file+0x24/0xf4
   #1:  (dev_base_lock){++.?..}, at: [<c12844ee>] wireless_show+0x1a/0x4c
  Pid: 10450, comm: head Not tainted 2.6.32-rc3 #1
  Call Trace:
   [<c102301c>] __might_sleep+0xf0/0xf7
   [<c1324355>] mutex_lock_nested+0x1a/0x33
   [<f8cea53b>] wdev_lock+0xd/0xf [cfg80211]
   [<f8cea58f>] cfg80211_wireless_stats+0x45/0x12d [cfg80211]
   [<c13118d6>] get_wireless_stats+0x16/0x1c
   [<c12844fe>] wireless_show+0x2a/0x4c

Fix this by using the rtnl instead of dev_base_lock.

Reported-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>