bl_free_block_dev() may sleep. We can not call it with spinlock held.
Besides, there is no need to take bm_lock as we are last user freeing bm_devlist.

Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: NPeng Tao <peng_tao@emc.com>
Signed-off-by: NBenny Halevy <bhalevy@tonian.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>