Change VOP_READDIR's cookies argument to a **uint64_t

The cookies argument is only used by the NFS server. NFSv2 defines the
cookie as 32 bits on the wire, but NFSv3 increased it to 64 bits. Our
VOP_READDIR, however, has always defined it as u_long, which is 32 bits
on some architectures. Change it to 64 bits on all architectures. This
doesn't matter for any in-tree file systems, but it matters for some
FUSE file systems that use 64-bit directory cookies.

PR: 260375
Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D33404