nfs: remove WARN_ON_ONCE from nfs_direct_good_bytes

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: <stable@vger.kernel.org> # v3.19+
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>