md.c:didn't unlock the mddev before return EINVAL in array_size_store

md.c: it needs to release the mddev lock before
the array_size_store() returns.

Fixes: ab5a98b1 ("md-cluster: change array_sectors and update size are not supported")

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Reviewed-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>