restart: fix corrupted restart in some scenarios
restart: fix corrupted restart in some scenarios

If the mmap file is reused but the memory isn't supposed to be reused,
pages are thrown into the global page pool. Normally when pages are
released into the pool the header of the page is zero'ed so the
restart_check() code will know to place it back into the global pool.

When restarting multiple times the slabs_prefill() part of the startup
code was missing this zero'ing step, so the _next_ time restart happens
properly restart_check() could attempt to recover that memory.

dormando/memcached,dormando/memcached,memcached/memcached,memcached/memcached,dormando/memcached,dormando/memcached,memcached/memcached,dormando/memcached,memcached/memcached,memcached/memcached