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

This reverts commit 672c3b74.

fuse_writepages() might be called with no dirty pages after all writable
opens were closed.  In this case __fuse_write_file_get() will return NULL
which will trigger the WARNING.

The exact conditions under which this is triggered is unclear and syzbot
didn't find a reproducer yet.

Reported-by:  <syzbot+217a976dc26ef2fa8711@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/all/CAJnrk1aQwfvb51wQ5rUSf9N8j1hArTFeSkHqC_3T-mU6_BCD=A@mail.gmail.com/


Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>