Fix the pNFS client when mirrors aren't on the same machine.

Without this patch, the client side NFSv4.1 pNFS code erroneously did writes
and commits to both DS mirrors using the TCP connection of the first one.
For my test setup this worked, since I have both DSs running on the same
machine, but it would have failed when the DSs are on separate machines.
This patch fixes the code to use the correct TCP connection for each DS.
This patch should only affect the NFSv4.1 client when using "pnfs" mounts
to mirrored DSs.

MFC after: 2 weeks