poolmanager: Fix full pool detection for WASS

When a file cannot fit within the free plus removable space without
crossing the pool gap, a pool is considered full. WASS erroneously
used the decayed removable space in this check. This meant that
if all pools had very fresh files, they could all be considered
full even though there are plenty of removable files. This patch
changes the check to consider all removable space of a pool. The
decayed removable space is only used when weighting the random
selection.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8390
Require-notes: yes
Require-book: no
Acked-by: Christian Bernardt <christian.bernardt@desy.de>
Patch: https://rb.dcache.org/r/7531/
(cherry picked from commit 42e31b5f935af4f3220aa29f2e51e8ea2c0bd146)
Merge pull request #1147 from gbehrmann/fix/2.8/rb7531

poolmanager: Fix full pool detection for WASS