btrfs: mark btrfs_put_caching_control() static

btrfs_put_caching_control() is only used in block-group.c, so mark it
static.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Lijuan Li <lilijuan@iscas.ac.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>