nfs: split nfs_read_folio

nfs_read_folio is a bit hard to follow because it mixes highlevel logic
with the actual data read.  Split the latter into a helper and update
the comments to be more accurate.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>