vmem: do not reinitialize the dss_mtx mutex

The dss_mtx mutex is locked at chunk_dss.c:50
(called from malloc_init_base_pool() at jemalloc.c:362)
before being initialized at chunk_dss.c:167
(called from malloc_init_base_pool() at jemalloc.c:364).

Move call to chunk_dss_boot() to chunk_global_boot()
in order to fix this bug.

Ref: pmem/issues#16