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/