ZGC: Do not resize TLABs unless -XX:ResizeTLAB is enabled

ZGC currently calls ThreadLocalAllocBuffer::resize() regardless of if -XX:ResizeTLAB is enabled or not. This causes the following assert to fail:

# Internal Error (open/src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp:130), pid=22060, tid=22069
# assert(ResizeTLAB) failed: Should not call this otherwise