call_rcu: drop mutex

The mutex is being used only to protect OR accesses to the flags.
Just use atomic operations for that.

[ Edit: this also fixes busy-looping on flags that were previously read
without volatile access, which could lead to never-ending loop given the
appropriate set of compiler optimisations. ]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>