The rules for nd->root are messy:
	* if we have LOOKUP_ROOT, it doesn't contribute to refcounts
	* if we have LOOKUP_RCU, it doesn't contribute to refcounts
	* if nd->root.mnt is NULL, it doesn't contribute to refcounts
	* otherwise it does contribute

terminate_walk() needs to drop the references if they are contributing.
So everything else should be careful not to confuse it, leading to
rather convoluted code.

It's easier to keep track of whether we'd grabbed the reference(s)
explicitly.  Use a new flag for that.  Don't bother with zeroing
nd->root.mnt on unlazy failures and in terminate_walk - it's not
needed anymore (terminate_walk() won't care and the next path_init()
will zero nd->root in !LOOKUP_ROOT case anyway).

Resulting rules for nd->root refcounts are much simpler: they are
contributing iff LOOKUP_ROOT_GRABBED is set in nd->flags.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>