rallocm(ALLOCM_ZERO) only zeroes *new* memory

rallocm(ALLOCM_ZERO) only zeroes *new* memory

Summary:
If you allocate N bytes and then try to grow that in place using rallocm
(ALLOCM_ZERO | ALLOCM_NO_MOVE) to M > N, our code assumed that all
memory from N to M would be zeroed.  In fact, rallocm only zeroes *new*
memory, so if jemalloc actually allocated N1 > N bytes initially,
rallocm will only zero memory from N1 to M.

Fixed by using calloc.

Test Plan: thread_local_test, tested in production

Reviewed By: delong.j@fb.com

FB internal diff: D519781