CIFS: Implement cifs_strict_readv (try #4)

Read from the cache if we have at least Level II oplock - otherwise
read from the server. Add cifs_user_readv to let the client read into
iovec buffers.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Pavel Shilovsky <piastryyy@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

diff --cc fs/cifs/cifsfs.c
Simple merge

diff --cc fs/cifs/cifsfs.h
Simple merge

diff --cc fs/cifs/file.c
Simple merge