namei: make set_root_rcu() return void

namei: make set_root_rcu() return void

The only caller that cares about its return value can just
as easily pick it from nd->root_seq itself.  We used to just
calculate it and return to caller, but these days we are
storing it in nd->root_seq in all cases.

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