Fix reading of released memory

Fix reading of released memory

lvseg_segtype_dup used memory pool vg memory pool for strind duplication.
However this one gets released before reporting happens so the command like:

pvs -o segtype

prints data from already released memory pool. Thanks to the fact there
is not much allocation happing after the VG is released, the memory
stays unmodified and correct result is printed.

Fix adds support for mempool passed parameter (like other similar
query commands) and uses dm_report memory pool for string duplication.