zfs: Avoid a gcc -Wint-to-pointer-cast warning

On 32-bit platforms long long is generally 64-bits. Sufficiently modern
versions of gcc (13 in my testing) complains when casting a pointer to
an integer of a different width so cast to uintptr_t first to avoid the
warning.

Fix i386 gcc builds while we wait for this to be merged to OpenZFS.

Sponsored by: DARPA, AFRL
Pull Request: https://github.com/openzfs/zfs/pull/16203