LU-2775 fid: allow FID-on-OST in fid_seq_is_mdt()

The LASSERT_SEQ_IS_MDT() macro used fid_seq_is_mdt() in several
places to verify that a FID was "sane" for where it was being used.
However, there should never be LASSERTs for data from the network
or disk.

The use of LASSERT_SEQ_IS_MDT() is removed from clients, since this
is "validating" data from the network.  This macro is no longer used,
so remove it.  The old CMD objseq_to_mdsno() and mdt_to_obd_objseq()
helpers are also no longer needed for DNE, and can be removed.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I74bc9198799045b8bd91510cb45e8f876012cab0
Reviewed-on: http://review.whamcloud.com/5456
Tested-by: Hudson
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>