Fix LINT kernel builds on powerpc64 and sparc64. This is a direct commit
to 12-stable because the nandfs code no longer exists in 13-current.

The build was failing with

nandfs_dat.c:301:

warning: comparison is always false due to limited range of data type

I tried to fix it with an inline (size_t) cast of nargv->nv_nmembs in the
if() expression, but that didn't help (which seems a bit buggy), but using
an intermediate variable fixed it. Elegance doesn't matter as much as
suppressing the warning; this code is long-dead even on this branch.