rtentry: Initialize rt_mtx with MTX_NEW

The "rtentry" zone does not use UMA_ZONE_ZINIT, so it is invalid to assume the
mutex's memory will be zero. Without MTX_NEW, garbage backing memory may
trigger the "re-initializing a mutex" assertion.

PR: 200991
Submitted by: Chang-Hsien Tsai <luke.tw AT gmail.com>