net: lwm2m: increase rd client stack size when NET_LOG_GLOBAL=y

The stack of rd client is exhausted while running lwm2m client w/ IPv6
and network log global enabled. Increase the stack size to 1536 when
NET_LOG_GLOBAL is enabled.

Detail described at #4424

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>