NFS: Create a common generic_pg_pgios()

What we have here is two functions that look identical.  Let's share
some more code!

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

diff --cc fs/nfs/internal.h
Simple merge

diff --cc fs/nfs/pagelist.c
Simple merge

diff --cc fs/nfs/read.c
Simple merge

diff --cc fs/nfs/write.c
Simple merge