fuse: restructure fuse_readpage()

Move the code filling and sending read request to a separate function. Future
patches will use it for .write_begin -- partial modification of a page
requires reading the page from the storage very similarly to what fuse_readpage
does.

Change-Id: Ie02bdfd5dd7570f454d6d7cd9142b9be3e3198f1
Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Git-commit:  482fce55d2809d639fd0d2e6249c89dedc20eeae
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git


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