nfs: don't truncate directory cookies to 32-bits in the NFS server

In NFSv2, the directory cookie was 32-bits. NFSv3 widened it to
64-bits and SVN r22521 widened the corresponding argument in
VOP_READDIR, but FreeBSD's NFS server continued to treat the cookies as
32-bits, and 0-extended to fill the field on the wire. Nobody ever
noticed, because every in-tree file system generates cookies that fit
comfortably within 32-bits.

Also, have better type safety for txdr_hyper. Turn it into an inline
function that type-checks its arguments. Prevents warnings about
shift-count-overflow.

PR: 260375
MFC after: 2 weeks
Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D33404