pNFS/flexfiles: When mirrored, retry failed reads by switching mirrors

If the pNFS/flexfiles file is mirrored, and a read to one mirror fails,
then we should bump the mirror index, so that we retry to a different
mirror. Once we've iterated through all mirrors and all failed, we can
return the layout and issue a new LAYOUTGET.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>