btrfs: free-space-cache.c: remove extra semicolon.

The patch below removes an extra semicolon.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
CC: Chris Mason <chris.mason@oracle.com>
CC: linux-btrfs@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>