dentry_kill(): don't bother with retain_dentry() on slow path

We have already checked it and dentry used to look not worthy
of keeping.  The only hard obstacle to evicting dentry is
non-zero refcount; everything else is advisory - e.g. memory
pressure could evict any dentry found with refcount zero.
On the slow path in dentry_kill() we had dropped and regained
->d_lock; we must recheck the refcount, but everything else
is not worth bothering with.

Note that filesystem can not count upon ->d_delete() being
called for dentry - not even once.  Again, memory pressure
(as well as d_prune_aliases(), or attempted rmdir() of ancestor,
or...) will not call ->d_delete() at all.

So from the correctness point of view we are fine doing the
check only once.  And it makes things simpler down the road.

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