MFC r287280 (by delphij):
In r286705 (Illumos 5960/a2cdcdd), a separate thread is created with curproc
as parent. In the case of a send or receive, the curproc would be the
userland application that issues the ioctl. This would trigger an assertion
failure introduced in Solaris compatibility shims in r196458 when kernel is
compiled with INVARIANTS.

Fix this by using p0 (proc0 or kernel) as the parent thread when creating
the kernel threads.