btrfs: rename bitmap_set_bits() -> btrfs_bitmap_set_bits()

bitmap_set_bits() does not start with the FS' prefix and may collide
with a new generic helper one day. It operates with the FS-specific
types, so there's no change those two could do the same thing.
Just add the prefix to exclude such possible conflict.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>