exec: Add missing rcu_read_unlock

rcu_read_unlock was not called if the address_space_access_valid result is
negative.

This caused (at least) a problem when qemu on PPC/E500+TAP failed to terminate
properly and instead got stuck in a deadlock.

Signed-off-by: Roman Kapl <rka@sysgo.com>
Message-Id: <20170109110921.4931-1-rka@sysgo.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>