FreeBSD: Update argument types for VOP_READDIR

A recent commit to FreeBSD changed the type of
vop_readdir_args.a_cookies to a uint64_t**. There is no functional
impact to ZFS because ZFS only uses 32-bit cookies, which will be
zero-extended to 64-bits by the existing code.

https://github.com/freebsd/freebsd-src/commit/b214fcceacad6b842545150664bd2695c1c2b34f

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #12874