ovl: enable RCU'd ->get_acl()

Overlayfs does not cache ACL's (to avoid double caching).  Instead it just
calls the underlying filesystem's i_op->get_acl(), which will return the
cached value, if possible.

In rcu path walk, however, get_cached_acl_rcu() is employed to get the
value from the cache, which will fail on overlayfs resulting in dropping
out of rcu walk mode.  This can result in a big performance hit in certain
situations.

Fix by calling ->get_acl() with rcu=true in case of ACL_DONT_CACHE (which
indicates pass-through)

Reported-by: garyhuang <zjh.20052005@163.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

include/linux/fs.h

@@ -581,6 +581,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
 
 struct posix_acl;
 #define ACL_NOT_CACHED ((void *)(-1))
+/*
+ * ACL_DONT_CACHE is for stacked filesystems, that rely on underlying fs to
+ * cache the ACL.  This also means that ->get_acl() can be called in RCU mode
+ * with the LOOKUP_RCU flag.
+ */
 #define ACL_DONT_CACHE ((void *)(-3))
 
 static inline struct posix_acl *