lsm: convert security_setselfattr() to use memdup_user()

As suggested by the kernel test robot, memdup_user() is a better
option than the combo of kmalloc()/copy_from_user().

Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Acked-by: Casey Schaufler <[e-mail]>
Signed-off-by: Paul Moore <[e-mail]>

security/security.c

···
 	if (size > PAGE_SIZE)
 		return -E2BIG;
 
-	lctx = kmalloc(size, GFP_KERNEL);
-	if (lctx == NULL)
-		return -ENOMEM;
-
-	if (copy_from_user(lctx, uctx, size)) {
-		rc = -EFAULT;
-		goto free_out;
-	}
+	lctx = memdup_user(uctx, size);
+	if (IS_ERR(lctx))
+		return PTR_ERR(lctx);
 
 	if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
 	    lctx->len < lctx->ctx_len + sizeof(*lctx)) {