From: Alex Tomas <bzzz@tmi.comex.ru>

We have a race wherein the block allocator can decide that
journal_head.b_committed_data is present and then will use it.  But kjournald
can concurrently free it and set the pointer to NULL.  It goes oops.

We introduce per-buffer_head "spinlocking" based on a bit in b_state.  To do
this we abstract out pte_chain_lock() and reuse the implementation.

The bit-based spinlocking is pretty inefficient CPU-wise (hence the warning
in there) and we may move this to a hashed spinlock later.