nfs: remove spurious WARN_ON_ONCE in write path

This WARN_ON_ONCE was supposed to catch reference counting bugs, but can
trigger in inappropriate situations.

This was reproducible using NFSv2 on an architecture with 64K pages -- we
verified that it was not a reference counting bug and the warning was
safe to ignore.

Reported-by: Will Deacon <will.deacon@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 1249384..f83b02d 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -715,8 +715,6 @@
 
 	if (test_and_clear_bit(PG_INODE_REF, &req->wb_flags))
 		nfs_release_request(req);
-	else
-		WARN_ON_ONCE(1);
 }
 
 static void