srm: Fix reporting of infinite space lifetime

A lifetime of -1 indidicates that the space has an infinite lifetime. This
needs to be taken into account when converting between seconds and milli-
seconds.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7269/
(cherry picked from commit 3273e4ac85c2e0c22cbf9d1299beecd89806c0e8)
Merge pull request #823 from gbehrmann/fix/2.10/rb7269

srm: Fix reporting of infinite space lifetime