MFC r274303:

Apply upstream 13597:3eac1e8e0f4c (git: illumos-gate@aa846ad9):

Initialize tqent_flags in the userland taskq implementation. Without
this the assertion of tq->tq_freelist != NULL may fail in taskq_destroy.

The problem is that tqent_flags is never initialized in the userland
implementation while the kernel one does initialize it. Without proper
initialization, the flag may have its lowest bit set, making it treated
as TQENT_FLAG_PREALLOC and never removing taskq_ent_t from tq_freelist.