Grab the kernel lock in uvm_wxcheck() when aborting the process

Grab the kernel lock in uvm_wxcheck() when aborting the process

kern.wxabort=1 logs and kills programs after W^X violations.
At least sigexit() -> coredump() as well as the non-atomic increment of
ps_wxcounter require protection, so grab the big lock for the entire block.

This is part of the effort to unlock mmap(2)'s MAP_ANON case.

Feedback mvs claudio kettenis deraadt
OK kettenis