teach set_nameidata() to handle setting the root as well

That way we don't need the callers to mess with manually setting any fields
of nameidata instances.  Old set_nameidata() gets renamed (__set_nameidata()),
new becomes an inlined helper that takes a struct path pointer and deals
with setting nd->root and putting ND_ROOT_PRESET in nd->state when new
argument is non-NULL.

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