fuse: support clients that don't implement 'open'

open/release operations require userspace transitions to keep track
of the open count and to perform any FS-specific setup.  However,
for some purely read-only FSs which don't need to perform any setup
at open/release time, we can avoid the performance overhead of
calling into userspace for open/release calls.

This patch adds the necessary support to the fuse kernel modules to prevent
open/release operations from hitting in userspace. When the client returns
ENOSYS, we avoid sending the subsequent release to userspace, and also
remember this so that future opens also don't trigger a userspace
operation.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/fuse_i.h

@@ -485,6 +485,9 @@ struct fuse_conn {
 	 * and hence races in setting them will not cause malfunction
 	 */
 
+	/** Is open/release not implemented by fs? */
+	unsigned no_open:1;
+
 	/** Is fsync not implemented by fs? */
 	unsigned no_fsync:1;