nvme: Allow larger user request sizes

We have a small buffer for pages on the stack, but if the user wants to
do an I/O larger than this we currently fail w/o a way for the user to
know the max size. It's not hard to allocate an array for the uncommon
case of very large I/Os, and the performance advantage of the array is
small in that case anyway. In addition, this allows firmware upgrades
using the full transfer size of the device as a happy accident too.

Sponsored by: Netflix
Reviewed by: chs, chuck
Differential Revision: https://reviews.freebsd.org/D55638