NFS: Fix initialization of nfs_page_array->npages

Commit 8ef9b0b9e1c0 open-coded nfs_pgarray_set(), and left out the
initialization of the nfs_page_array's npages.  This mistake didn't show up
until testing with block layouts, and there shows that all pNFS reads
return -EIO.

Fixes: 8ef9b0b9e1c0 ("NFS: move nfs_pgarray_set() to open code")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Cc: stable@vger.kernel.org # 4.12
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/pagelist.c

@@ -779,6 +779,7 @@ int nfs_generic_pgio(struct nfs_pageio_descriptor *desc,
 	gfp_t gfp_flags = GFP_KERNEL;
 
 	pagecount = nfs_page_array_len(mirror->pg_base, mirror->pg_count);
+	pg_array->npages = pagecount;
 
 	if (pagecount <= ARRAY_SIZE(pg_array->page_array))
 		pg_array->pagevec = pg_array->page_array;