NFS: Allow the nfs_pageio_descriptor to signal that a re-coalesce is needed

If an attempt to do pNFS fails, and we have to fall back to writing through
the MDS, then we may want to re-coalesce the requests that we already have
since the block size for the MDS read/writes may be different to that of
the DS read/writes.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>