Resolve a race when physio_done signals completion before it tries to
free a buffer.  This will fail if the buffer owner has a chance to
modify the BC_DONTFREE flag before putphysbuf() examines it.

Fix by removing get/putphysbuf() and BC_DONTFREE.  Physio_done() now
has an explicit test for a buffer coming from the call of physio().

Observed by Lars Nordlund when writing a DVD with growisofs, see PR kern/39536.

Reviewed by: Jason Thorpe <thorpej@netbsd.org>

(hannken)