kbind(2): unlock syscall, push kernel lock down to binding loop

kbind(2): unlock syscall, push kernel lock down to binding loop

- Rearrange the security check code in sys_kbind() so that we only
  need to take the kernel lock once if we need to raise SIGILL.

- Protect process.ps_kbind_addr and process.ps_kbind_cookie with
  process.ps_mtx.  This is easier to do after the aforementioned
  rearrangement.  Under normal circumstances this isn't necessary:
  the process is single-threaded when we initialize kbind(2).
  But in stranger situations this brief mutex ensures that the
  first thread to reach sys_kbind() initializes both variables.

- Wrap the binding loop with the kernel lock.  We need to carefully
  confirm that uvm_unmap_remove(), uvm_map_extract(), and
  uvm_unmap_detach() are MP-safe in a subsequent patch before
  completely removing the kernel lock from sys_kbind().

- Remove the kernel lock from kbind(2) in syscalls.master.

Prompted by mpi@, dlg@, and deraadt@.  Current patch workshopped with
deraadt@.  Based on a patch from dlg@.

With input from dlg@, bluhm@, mpi@, kettenis@, deraadt@, and
guenther@.

Thread: https://marc.info/?l=openbsd-tech&m=165274831829349&w=2

ok deraadt@ kettenis@ mpi@