ctl_backend_ramdisk: Add support for NVMe

One known caveat is that the support for WRITE_UNCORRECTABLE is not
quite correct as reads from LBAs after a WRITE_UNCORRECTABLE will
return zeroes rather than an error. Fixing this would likely require
special handling for PG_ANCHOR for NVMe requests (or adding a new
PG_UNCORRECTABLE).

Reviewed by: ken, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44722