userret: assert td_lk_slocks == 0

I am trying to debug a leak of a shared vnode lock and I noticed that
there is no check for td_lk_slocks in userret.  There are checks for
td_rw_rlocks and td_sx_slocks.  I wonder if there is any valid scenario
where a thread is allowed to retain a shared lock manager lock across
system calls.

Thanks!
-- 
Andriy Gapon