vfs: get rid of 'struct dcache_hash_bucket' abstraction

It's a useless abstraction for 'hlist_bl_head', and it doesn't actually
help anything - quite the reverse.  All the users end up having to know
about the hlist_bl_head details anyway, using 'struct hlist_bl_node *'
etc. So it just makes the code look confusing.

And the cost of it is extra '&b->head' syntactic noise, but more
importantly it spuriously makes the hash table dentry list look
different from the per-superblock DCACHE_DISCONNECTED dentry list.

As a result, the code ended up using ad-hoc locking for one case and
special helper functions for what is really another totally identical
case in the very same function.

Make it all look and work the same.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>