Now that fs.h grow due to the lock.h removal let's reduce it's overhead
again:  Instead of penalizing ever user of fs.h with the overhead of the
buffer head interface let it's users include it directly.

This also shows nicely which parts of the core kernel still depend on the
buffer head interface, and allows that to be cleaned up properly.

This is the first of ten patches and adds the includes needed by
buffer_head.h to it and fixes it's inclusion guard.