nfs: count DIO good bytes correctly with mirroring
nfs: count DIO good bytes correctly with mirroring

When resending to MDS, we might resend multiple mirroring
requests to MDS. As a result, nfs_direct_good_bytes() ends
up counting bytes multiple times, causing application to
get wrong return results in read/write syscalls.

Fix it by tracking start of a dreq and checking the range of
pgio header.

Cc: Weston Andros Adamson <2782014720c8dba08f7eb23e9f1d190c445c0c94@primarydata.com>
Signed-off-by: Peng Tao <4fb2b5cc20694ed9d470f738b651303a1e636b2d@primarydata.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs