Correct initialization of the cache doubly-linked list

Correct initialization of the cache doubly-linked list

The initialization of the cache doubly-linked list had
head->next->prev instead of head->prev->next; this entry is supposed
to initialize the ->next entry of the last entry in the list (which
points back to the head node.)

For clarity, consistently use "head" to refer to the head node; the
mixing of "head" and "dev->cache_head" needlessly obfuscated the code.

The wild pointer reference caused crashes on some systems.

Reported-by: Jan Safrata <jsafrata@centrum.cz>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>