fuse: redundant get_fuse_inode() calls in fuse_writepages_fill()

Currently fuse_writepages_fill() calls get_fuse_inode() few times with
the same argument.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>