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

[ Upstream commit 360f54796ed65939093ae373b92ebd5ef3341776 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>