Fix the searches of the device id lists so that they include the mirror
entries. Also, modify nfsrv_dssetsockmnt() so that it only returns ENOENT
if none of the DSs were found. This will allow a mirrored system to run in
a degraded mode when one mirror is offline.

The mirroring is now working for normal operation, although Flex File layout
has not yet been tested (all I/O goes through MDS when mirroring is set up).

Now, the fun part is to code handling of a DS mirror failure and resilvering
of a DS mirror to bring it back online.
Also, testing of Flex File layout against a recent Linux client needs to be
done.