fuse: factor out helper fuse_truncate_update_attr()

fuse_finish_open() is called from fuse_open_common() and from
fuse_create_open().  In the latter case, the O_TRUNC flag is always
cleared in finish_open()m before calling into fuse_finish_open().

Move the bits that update attribute cache post O_TRUNC open into a
helper and call this helper from fuse_open_common() directly.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>