jbd2: Add BH_JBDPrivateStart

(cherry picked from commit e97fcd95a4778a8caf1980c6c72fdf68185a0838)

Add this so that file systems using JBD2 can safely allocate unused b_state
bits.

In this case, we add it so that Ocfs2 can define a single bit for tracking
the validation state of a buffer.

Acked-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>