fuse: Prepare to handle short reads

A helper which gets called when read reports less bytes than was requested.
See patch "trust kernel i_size only" for details.

Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>