MFC r271747:
 - Use NULL instead of 0 for fpcurthread.
 - Note the quirk with the interrupt enabled state of the dna handler.
 - Use just panic() instead of printf() and panic().  Print tid instead
   of pid, the fpu state is per-thread.

MFC r271924:
Update and clarify comments.  Remove the useless counter for impossible, but
seen in wild situation (on buggy hypervisors).
MFC r272130:
In kern_linkat() and kern_renameat(), do not call namei(9) while
holding a write reference on the filesystem.  Try to get write
reference in unblocked way after all vnodes are resolved; if failed,
drop all locks and retry after waiting for suspension end.