adjust calling conventions of lock_for_kill(), fold __dentry_kill() into dentry_kill()

Pull dropping ->d_lock on lock_for_kill() failure into lock_for_kill() itself.
That reduces dentry_kill() to
	if (!lock_for_kill(dentry))
		return NULL;
	return __dentry_kill(dentry);
at which point it's easier to move that if (...) into the beginning of __dentry_kill()
itself and rename it into dentry_kill().

Document the new calling conventions of lock_for_kill().

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