Previously, deallocuvm scanned from 0 to KERNBASE in one page
increments, which had a noticable effect on boot time.  Now it skips
over missing page directories.

Conflicts:
	vm.c

exec() with arguments that don't fit on a single page.

My changes have a race with re-used bufs and the code doesn't seem to get shorter
Keep the changes that fixed ip->off race

This reverts commit 3a5fa7ed.

Conflicts:

	defs.h
	file.c
	file.h

Remove I_BUSY, B_BUSY, and intrans defs and usages
One spinlock per buf to avoid ugly loop in bget
fix race in filewrite (don't update f->off after releasing lock)