spa: avoid type narrowing warning

Building the spa module for i386 caused gcc to emit
-Wint-to-pointer-cast "cast to pointer from integer of different size"
because spa.spa_did was uint64_t but pthread_join (via thread_join in
spa_deactivate) takes a pointer (32-bit on i386). Define spa_did to be
pointer-size instead. For now spa_did is in fact never non-zero and the
thread_join could instead be ifdef'd out, but changing the size of
spa_did may be more useful for the future.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #11336