ktls: re-work alloc thread

When the ktls_buffer zone needs to expand, it may fail due
to a lack of physically contiguous memory. We tried to rectify
that by introducing an alloc thread to provide a context where
it is harmless to sleep, and letting that thread repopulate
the ktls_buffer zone.

However, it turns out that M_WAITOK is not enough, and we
must call vm_page_reclaim_contig_domain() to reclaim contig
memory. Worse, M_WAITOK results in the allocation essentially
busy-looping around vm_domain_alloc_fail() returning EAGIN,
causing vm_page_alloc_noobj_contig_domain() to loop and resulting
in the alloc thread consuming 100% CPU.

To fix this, we change the alloc thread to call
vm_page_reclaim_contig_domain_ext()

In order to prevent the busy loop around vm_domain_alloc_fail(), we
must change the uma_zalloc flags to M_NORECLAIM | M_NOWAIT. However,
once that is done, these allocations become no different than the
allocations done in the critical path in ktls_buffer_alloc(), so its
best to just eliminate them.

Since we're no longer doing allocations but just calling
vm_page_reclaim_contig_domain_ext(), the name has changed to the ktls
reclaim thread.

Reviewed by: jhb, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D39421