dcache: let the dentry count go down to zero without taking d_lock

We can be more aggressive about this, if we are clever and careful. This is subtle.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/dcache.c
Simple merge

diff --cc include/linux/lockref.h
Simple merge

diff --cc lib/lockref.c
Simple merge