an I/O attempt on a DS to the server via LayoutReturn.

The current FreeBSD client can generate these errors for an operational
DS while doing a recovery of a mirror after a mirrored DS has been repaired.
I am not sure why these errors occur, but my best current guess is a race
between the Layout Recall issued by the kernel code run from pnfsdscopymr(8)
and a Read operation on the DS for the file bing copied.
The errrors are not fatal, since the client falls back on doing I/O through
the MDS, which can do the I/O successfully as a proxy. (The fact that the
MDS can do this indicates that the file does still exist on the functioning
DS.)
This patch only affects behaviour of the pNFS client and only when using
Flexible File layouts.

MFC after:	2 weeks