vfs_aio.c: rename "physio" symbols to "bio".

aio has two paths: an asynchronous "physio" path and a synchronous path.
Confusingly, physio(9) isn't actually used by the "physio" path, and never
has been. In fact, it may even be called by the synchronous path! Rename
the "physio" path to the "bio" path to reflect what it actually does:
directly compose BIOs and send them to character devices.

MFC after: 2 weeks