merge path_init and path_init_rcu

Actual dependency on whether we want RCU or not is in 3 small areas
(as it ought to be) and everything around those is the same in both
versions.  Since each function has only one caller and those callers
are on two sides of if (flags & LOOKUP_RCU), it's easier and cleaner
to merge them and pull the checks inside.

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