fuse: make fuse_direct_io() aware about AIO

The patch implements passing "struct fuse_io_priv *io" down the stack up to
fuse_send_read/write where it is used to submit request asynchronously.
io->async==0 designates synchronous processing.

Non-trivial part of the patch is changes in fuse_direct_io(): resources
like fuse requests and user pages cannot be released immediately in async
case.

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index be5c7e1..07bdb14 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -111,7 +111,7 @@
 	sigprocmask(SIG_SETMASK, oldset, NULL);
 }
 
-static void __fuse_get_request(struct fuse_req *req)
+void __fuse_get_request(struct fuse_req *req)
 {
 	atomic_inc(&req->count);
 }