fs/dcache: bring back explicit INIT_HLIST_BL_HEAD init

Commit 3d375d78 ("mm: update callers to use HASH_ZERO flag") removed
INIT_HLIST_BL_HEAD and uses the ZERO flag instead for the init. However
on RT we have also a spinlock which needs an init call so we can't use
that.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>