acl: use uintptr_t for ace walker cookies

Avoid assuming that a pointer can fit in a uint64_t and use uintptr_t
instead.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14131