linux: Enable NFSv4.1, v4.2 clients and swap on NFS

I'm only enabling for kernels >= 3.11 to be conservative, because clients and
servers automatically negotiate and use the highest mutually supported version
by default, but only in kernel 3.11 server NFSv4.1 support actually became RFC
compliant.

I'm also adding support for swap on NFS, which is enabled by default on
Ubuntu kernels.