fuse: Implement write_begin/write_end callbacks

The .write_begin and .write_end are requiered to use generic routines
(generic_file_aio_write --> ... --> generic_perform_write) for buffered
writes.

Change-Id: I37480cf01898a6692eb410e1998a0a80e906dcc5
Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Git-commit:  6b12c1b37e5556af073c1ebfa04c1f9df3a2beaf
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git


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