fuse: allow batching of FORGET requests

Terje Malmedal reports that a fuse filesystem with 32 million inodes
on a machine with lots of memory can take up to 30 minutes to process
FORGET requests when all those inodes are evicted from the icache.

To solve this, create a BATCH_FORGET request that allows up to about
8000 FORGET requests to be sent in a single message.

This request is only sent if userspace supports interface version 7.16
or later, otherwise fall back to sending individual FORGET messages.

Reported-by: Terje Malmedal <terje.malmedal@usit.uio.no>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/fuse_i.h

@@ -55,8 +55,7 @@ extern unsigned max_user_congthresh;
 
 /* One forget request */
 struct fuse_forget_link {
-	u64	nodeid;
-	u64	nlookup;
+	struct fuse_forget_one forget_one;
 	struct fuse_forget_link *next;
 };