Chase r321920 and r321930 (dev_t being widened)

The layout of st_rdev has changed after this commit, and assumptions made
in the NetBSD tests are no longer valid. Change the hardcoded assumed
values to account for the fact that major/minor are now represented by
64 bits as opposed to the less precise legacy precision of 16 bits.

PR: 221048
Relnotes: st_rdev layout changed; warning about impact of r321920 to

	  downstream consumers