nvme: change namei_request_zone into a malloc type

Both the size (128 bytes) and ephemeral nature of allocations make it a great
fit for malloc.

A dedicated zone unnecessarily avoids sharing buckets with 128-byte objects.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D27103

(cherry picked from commit 71460dfcb275f0a2a20b39a332b0e1149c6e7e3f)