nvme: Use size_t for payload size

Using uint32_t here can cause spurious compiler warnings. I assume it
was done to constrain the range; use a KASSERT instead.

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D55279

(cherry picked from commit be7e4dc878eab090cd411c9ef28880e4f62116f5)