afs: Calculate lock extend timer from set/extend reply reception

afs: Calculate lock extend timer from set/extend reply reception

Record the timestamp on the first reply DATA packet received in response to
a set- or extend-lock operation, then use this to calculate the time
remaining till the lock expires rather than using whatever time the
requesting process wakes up and finishes processing the operation as a
base.

Signed-off-by: David Howells <dhowells@redhat.com>