Introduce RCU-enabled DQs.

Add RCU-enabled variants on the existing bsd DQ facility. Each Q
operation has the same interface as the existing (non-RCU)
version. Also, each operation is implemented as macro for now.

Using the RCU-enabled DQ, existing DQ users will be able to convert to
RCU without using a different list interface.

To accompany the RCU-enabled DQ, there is also a test file that uses
concurrent readers to contend with a single updater.

This patchset builds on top of Paolo Bonzini's rcu tree.

Signed-off-by: Mike Day <ncmike@ncultra.org>