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.

Change-Id: Ie62ca83066d7a91cb1fa4920f8ef822542711f1c
Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Git-commit:  a92adc824ed5feaa2d4f7029f21170f574987aee
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>