NFS: Don't rely on PageError in nfs_readpage_release_partial

Don't rely on the PageError flag to tell us if one of the partial reads of
the page failed. Instead, replace that with a dedicated flag in the
struct nfs_page.

Then clean out redundant uses of the PageError flag: the VM no longer
checks it for reads.

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

include/linux/nfs_page.h

@@ -34,6 +34,7 @@ enum {
 	PG_NEED_COMMIT,
 	PG_NEED_RESCHED,
 	PG_PNFS_COMMIT,
+	PG_PARTIAL_READ_FAILED,
 };
 
 struct nfs_inode;