fuse: Add missed unlock_page() to fuse_readpages_fill()

The above error path returns with page unlocked, so this place seems also
to behave the same.

Fixes: f8dbdf81 ("fuse: rework fuse_readpages()")
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>