vmem: fix freeing memory by iqalloct()

In case of huge pages iqalloct() calls huge_dalloc() for the pool #0
(src/jemalloc/include/jemalloc/internal/jemalloc_internal.h.in:834),
but huge_dalloc() should be called with the pool 'arena->pool'
as an argument in functions huge_ralloc(), arena_ralloc()
and iralloct_realign().

Add two functions: pool_iqalloct() and pool_idalloct() in order
to be able to pass a 'pool' argument from pool_iqalloct()
to huge_dalloc().

Ref: pmem/issues#8