nfs: clean up nfs_direct_IO

This follows up "nfs: fix dio deadlock when O_DIRECT flag is flipped"
and removes the unnecessary CONFIG_NFS_SWAP switch.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>