If a read-write layout has an invalid mirror, then we should
mark it as invalid, and return it.
If a read-only layout has an invalid mirror, then mark it as invalid
and check if there is still at least one valid mirror before we return
it.

Note: Also fix incorrect use of pnfs_generic_mark_devid_invalid().
We really want nfs4_mark_deviceid_unavailable().
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>