This patch changes each mb_cache's both block and index hash chains to
use a hlist_bl_node, which contains a built-in lock.  This is the
first step in decoupling of locks serializing accesses to mb_cache
global data and each mb_cache_entry local data.
Signed-off-by: NT. Makphaibulchoke <tmac@hp.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>