nfs: fix DIO good bytes calculation

nfs: fix DIO good bytes calculation

For direct read that has IO size larger than rsize, we'll split
it into several READ requests and nfs_direct_good_bytes() would
count completed bytes incorrectly by eating last zero count reply.

Fix it by handling mirror and non-mirror cases differently such that
we only count mirrored writes differently.

This fixes 5fadeb47("nfs: count DIO good bytes correctly with mirroring").

Reported-by: Jean Spector <[email protected]>
Cc: <[email protected]> # v3.19+
Signed-off-by: Peng Tao <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>