[PowerPC64] fix uma_small_free panic

[alfredo]

Summary

This fixes panic like "Freeing UMA block at 0xc00000004b0ce000 with no associated page", reproduced when huge pages is not enabled and system starts using swap.

Solution provided by jhibbits.

Test Plan

Force swap usage running several buildworld threads as well with stress util on powerpc64 QEMU VM.

[jhibbits]

Given what I see in uma_small_alloc(), this change really should be unnecessary (uma_small_alloc() returns an identity-mapped page), but relying on that is obviously incorrect. The pmap_kextract() in here is the correct solution.

sys/powerpc/powerpc/uma_machdep.c

97–98

I think this should be pmap_kremove() anyway, to match the pmap_kenter() above. Semantically the same, but better symmetry.

[alfredo]

replaced pmap_extract by pmap_kextract

[alfredo]

System was running under load with latest change for almost 13 hours. Everything seems good.