afs: Fix calculation of callback expiry time

Fix the calculation of the expiry time of a callback promise, as obtained
from operations like FS.FetchStatus and FS.FetchData.

The time should be based on the timestamp of the first DATA packet in the
reply and the calculation needs to turn the ktime_t timestamp into a
time64_t.

Fixes: c435ee34

 ("afs: Overhaul the callback handling")
Signed-off-by: David Howells <dhowells@redhat.com>