new helper: complete_walk().  Done on successful completion
of walk, drops out of RCU mode, does d_revalidate of final
result if that hadn't been done already.

handle_reval_dot() and nameidata_drop_rcu_last() subsumed into
that one; callers converted to use of complete_walk().
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>