f2fs: use kfree() to free variables allocated by match_strdup()

Use kfree() instead of kvfree() to free variables allocated
by match_strdup(). Because the memory is allocated with kmalloc
inside match_strdup().

Signed-off-by: Wang Xiaojun <wangxiaojun11@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>