Btrfs: modify rw_devices counter under chunk_mutex context

rw_devices counter is often used to tune the profile when doing chunk allocation,
so we should modify it under the chunk_mutex context to avoid getting wrong
chunk profile.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>