f2fs: release locks before return in f2fs_ioc_gc_range()

Currently, we will leave the kernel with locks still held when the gc_range
is invalid. This patch fixes the bug.

Signed-off-by: Qiuyang Sun <sunqiuyang@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>