[FLINK-39924] Size jemalloc narenas from container CPU allowance

Apache Flink containers load jemalloc via LD_PRELOAD but don't configure
narenas. jemalloc's default is 4 * ncpus, where ncpus is read from
/proc/cpuinfo, the host CPU count, not the container's CPU limit. In
CPU limited pods on large hosts this over-provisions arenas and causes
RSS fragmentation, since each arena holds dirty pages for dirty_decay_ms
before releasing them to the OS.

Determine the effective CPU count from the cgroup CPU quota directly
(cpu.max for v2, cpu.cfs_quota_us / cpu.cfs_period_us for v1), since
nproc honors cpuset but not CPU quotas. Fall back to nproc when no quota
is set. Skip the override entirely when the user has supplied narenas
in MALLOC_CONF, and append narenas to any other user-supplied MALLOC_CONF
value.