aufs: for v5.1-rc1, minor, convert some BUILD_BUG_ON into static_assert

With a little hope to make it easier to read.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/dynop.c

@@ -341,9 +341,6 @@ void __init au_dy_init(void)
 {
 	int i;
 
-	/* make sure that 'struct au_dykey *' can be any type */
-	BUILD_BUG_ON(offsetof(struct au_dyaop, da_key));
-
 	for (i = 0; i < AuDyLast; i++)
 		INIT_HLIST_BL_HEAD(dynop + i);
 }

fs/aufs/dynop.h

@@ -44,6 +44,8 @@ struct au_dyaop {
 	struct au_dykey			da_key;
 	struct address_space_operations	da_op; /* not const */
 };
+/* make sure that 'struct au_dykey *' can be any type */
+static_assert(!offsetof(struct au_dyaop, da_key));
 
 /* ---------------------------------------------------------------------- */
 

fs/aufs/loop.c

@@ -116,7 +116,7 @@ int au_loopback_init(void)
 	int err;
 	struct super_block *sb __maybe_unused;
 
-	BUILD_BUG_ON(sizeof(sb->s_magic) != sizeof(unsigned long));
+	BUILD_BUG_ON(sizeof(sb->s_magic) != sizeof(*au_warn_loopback_array));
 
 	err = 0;
 	au_warn_loopback_array = kcalloc(au_warn_loopback_step,