btrfs: fix leak of fs devices after removing btrfs module

When removing the btrfs module we are not calling btrfs_cleanup_fs_uuids()
which results in leaking btrfs_fs_devices structures and other resources.
This is a regression recently introduced by a refactoring of the module
initialization and exit sequence, which simply removed the call to
btrfs_cleanup_fs_uuids() in the exit path, resulting in the leaks.

So fix this by calling btrfs_cleanup_fs_uuids() at exit_btrfs_fs().

Fixes: 5565b8e0 ("btrfs: make module init/exit match their sequence")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>