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

include/sys/zfs_onexit.h

@@ -54,7 +54,7 @@ extern void zfs_onexit_destroy(zfs_onexit_t *zo);
 extern zfs_file_t *zfs_onexit_fd_hold(int fd, minor_t *minorp);
 extern void zfs_onexit_fd_rele(zfs_file_t *);
 extern int zfs_onexit_add_cb(minor_t minor, void (*func)(void *), void *data,
-    uint64_t *action_handle);
+    uintptr_t *action_handle);
 
 #ifdef	__cplusplus
 }