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.

Fixes: c183d164aa Parallel pool import

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@klarasystems.com>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #16203