Revert "rcu: Permit rt_mutex_unlock() with irqs disabled"

This reverts commit 5342e269b2b58ee0b0b4168a94087faaa60d0567.

The approach taken in this patch was deemed too abusive to mutexes,
and thus too likely to result in maintenance problems in the future.
Instead, we will disallow RCU read-side critical sections that partially
overlap with interrupt-disbled code segments.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>