shmem: switch to iter_file_splice_write()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/shmem.c

@@ -2623,7 +2623,7 @@ static const struct file_operations shmem_file_operations = {
 	.write_iter	= generic_file_write_iter,
 	.fsync		= noop_fsync,
 	.splice_read	= shmem_file_splice_read,
-	.splice_write	= generic_file_splice_write,
+	.splice_write	= iter_file_splice_write,
 	.fallocate	= shmem_fallocate,
 #endif
 };