NFS: Code comments update in NFS

read_cache_mtime is no longer used in nfs_inode. This patch removes
references of read_cache_mtime in the code comments.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>