nfs: only reset desc->pg_mirror_idx when mirroring is supported

so that we don't reset desc->pg_mirror_idx for read unnecessarily.
Remove WARN_ON_ONCE from __nfs_pageio_add_request to allow LD to
set pg_mirror_idx for read where pg_mirror_count is always 1.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <loghyr@primarydata.com>
diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
index eec12b7..f9d8c46 100644
--- a/fs/nfs/pagelist.c
+++ b/fs/nfs/pagelist.c
@@ -1021,8 +1021,6 @@
 	unsigned int bytes_left = 0;
 	unsigned int offset, pgbase;
 
-	WARN_ON_ONCE(desc->pg_mirror_idx >= desc->pg_mirror_count);
-
 	nfs_page_group_lock(req, false);
 
 	subreq = req;
@@ -1162,7 +1160,8 @@
 		} else
 			dupreq = req;
 
-		desc->pg_mirror_idx = midx;
+		if (nfs_pgio_has_mirroring(desc))
+			desc->pg_mirror_idx = midx;
 		if (!nfs_pageio_add_request_mirror(desc, dupreq))
 			return 0;
 	}
@@ -1181,7 +1180,8 @@
 	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[mirror_idx];
 	u32 restore_idx = desc->pg_mirror_idx;
 
-	desc->pg_mirror_idx = mirror_idx;
+	if (nfs_pgio_has_mirroring(desc))
+		desc->pg_mirror_idx = mirror_idx;
 	for (;;) {
 		nfs_pageio_doio(desc);
 		if (!mirror->pg_recoalesce)