This should help prevent deadlocks where a thread blocks on a Mutex
while interrupts are disabled, and makes it impossible for the holder of
the Mutex to make forward progress because it cannot be scheduled in.
Hide it behind a new debug macro LOCK_IN_CRITICAL_DEBUG for now, because
Ext2FS takes a series of Mutexes from the page fault handler, which
executes with interrupts disabled.
The LOCK_DEBUG conditional code is pretty ugly for a feature that we
only use rarely. We can remove a significant amount of this code by
utilizing a zero sized fake type when not building in LOCK_DEBUG mode.
This lets us keep the same API, but just let the compiler optimize it
away when don't actually care about the location the caller came from.
