cleanup some odd uses of curproc. Remove PHOLD/PRELE around physical I/O

cleanup some odd uses of curproc.  Remove PHOLD/PRELE around physical I/O
(our UPAGES can no longer be swapped out and if they eventually are made
to it will only be when the thread is sleeping on a particular address).

Also move the inblock/oublock accounting in vfs_busy_pages() allowing us
to remove additional curproc references from various filesystem code.  This
also makes inblock/oublock more consistent.