In NetBSD 9.x and prior, udata is an intptr_t, but in 10.x (current
development branch) it was changed to be a void * for compatibility
with other BSDs a year or so ago.

Unfortunately, this does not simplify the code, as NetBSD 8.x and 9.x
are still supported and will be for a few more years.
Signed-off-by: Nia Alarie <nia@NetBSD.org>