[C++] ReleaseUnused does nothing with jemalloc due to invalid mallctl name #55362

Describe the bug, including details regarding any error messages, version, and platform.

The ReleaseUnused implementation for jemalloc calls:

mallctl("arena." ARROW_STRINGIFY(MALLCTL_ARENAS_ALL) ".purge", NULL, NULL, NULL, 0)

This is equivalent to calling mallctl with arena.MALLCTL_ARENAS_ALL.purge instead of the intended arena.4096.purge, because the preprocessor doesn't expand arguments that come directly after a # in the definition.

The return code for mallctl is ignored, but a debug check would have caught that this is returning an error in tests.

Component(s)

C++