fs: drop_caches: move sysctl to fs/drop_caches.c

The sysctl_drop_caches to fs/drop_caches.c, move it to
fs/drop_caches.c from /kernel/sysctl.c. And remove the
useless extern variable declaration from include/linux/mm.h

Signed-off-by: Kaixiong Yu <yukaixiong@huawei.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>