fuse: move fuse file initialization to wpa allocation time

fuse: move fuse file initialization to wpa allocation time

Before this change, wpa->ia.ff is initialized with an acquired reference
on the fuse file right before it submits the writeback request. If there
are auxiliary writebacks, then the initialization and reference
acquisition needs to also be set before we submit the auxiliary writeback
request.

To make the logic simpler and to pave the way for a subsequent
refactoring of fuse_writepages_fill() and fuse_writepage_locked(), this
change initializes and acquires wpa->ia.ff when the wpa is allocated.

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>