Don't attempt to lock the file descriptor in readv/writev if it's a socket
Don't attempt to lock the file descriptor in readv/writev if it's a socket

Summary:
There is no way to lock a socket on Windows, so these calls will always fail. Just don't do the lock on Windows.
To do it properly with sockets, this should probably be forwarding to `sendmsg`/`recvmsg`, but this is good enough for now.

Reviewed By: simpkins

Differential Revision: D5307901

fbshipit-source-id: 9855274e932a3e2ec3cacae10363200187e0c01b

Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,facebook/folly,Orvid/folly,Orvid/folly,rklabs/folly,Orvid/folly,facebook/folly,facebook/folly