nfs: remove WARN_ON_ONCE from nfs_direct_good_bytes

nfs: remove WARN_ON_ONCE from nfs_direct_good_bytes

commit 05f54903d9d370a4cd302a85681304d3ec59e5c1 upstream.

For flexfiles driver, we might choose to read from mirror index other
than 0 while mirror_count is always 1 for read.

Reported-by: Jean Spector <jean@primarydata.com>
Cc: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>