fuse: move initialization of fuse_file to fuse_writepages() instead of in callback

Prior to this change, data->ff is checked and if not initialized then
initialized in the fuse_writepages_fill() callback, which gets called
for every dirty page in the address space mapping.

This logic is better placed in the main fuse_writepages() caller where
data.ff is initialized before walking the dirty pages.

No functional changes added.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>