This is a follow-up commit to "slab: throw exception if initial
allocation fails". It is needed because on seL4, the PD session quota is
used as backing store for the meta data of the PD's virtual memory. For
this reason, PD sessions on seL4 require more session quota compared to
the other kernels. Apparently, the default quota is too scarce to
complete the PD-session construction. By shrinking the number of
supported page-table entries, we reduce the size of the PD session
component so that the creation succeeds.
Issue #1983
ebdb1c6892