This is the start of the JBD locking rework.

The aims of all this are to remove all lock_kernel() calls from JBD, to
remove all lock_journal() calls (the context switch rate is astonishing when
the lock_kernel()s are removed) and to remove all sleep_on() instances.




The strategy which is taken is:

a) Define the lcoking schema (this patch)

b) Work through every JBD data structure and implement its locking fully,
   according to the above schema.  We work from "innermost" data structures
   and outwards.

It isn't guaranteed that the filesystem will work very well at all stages of
this patch series.



In this patch:


Add commentary and various locks to jbd.h describing the locking scheme which
is about to be implemented.

Initialise the new locks.

Coding-style goodness in jbd.h