smk_write_relabel_self() frees memory from the task's credentials with
no locking, which can easily cause a use-after-free because multiple
tasks can share the same credentials structure.

Fix this by using prepare_creds() and commit_creds() to correctly modify
the task's credentials.

Reproducer for "BUG: KASAN: use-after-free in smk_write_relabel_self":

	#include <fcntl.h>
	#include <pthread.h>
	#include <unistd.h>

	static void *thrproc(void *arg)
	{
		int fd = open("/sys/fs/smackfs/relabel-self", O_WRONLY);
		for (;;) write(fd, "foo", 3);
	}

	int main()
	{
		pthread_t t;
		pthread_create(&t, NULL, thrproc, NULL);
		thrproc(NULL);
	}

Reported-by: syzbot+e6416dabb497a650da40@syzkaller.appspotmail.com
Fixes: 38416e53 ("Smack: limited capability for changing process label")
Cc: <stable@vger.kernel.org> # v4.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>