zfs_onexit_add_cb: make action_handle point to a uintptr_t

Avoid assuming than a uint64_t can hold a pointer and reduce the
number of casts in the process.

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