Modify sinval so that InvalidateSharedInvalid() does not hold

Modify sinval so that InvalidateSharedInvalid() does not hold
the SInval spinlock while it is calling the passed invalFunction or
resetFunction.  This is necessary to avoid deadlock with lmgr change;
InvalidateSharedInvalid can be called recursively now.  It should be
a good performance improvement anyway --- holding a spinlock for more
than a very short interval is a no-no.