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)

Without this, every write to the console results in log writes.

usertest for big write()s
push begin_trans/commit_trans down into syscalls

Passes usertests and stressfs
Seems to recover correctly in a number of simple cases