Enable WRITE SAME support in dm multipath.  As far as multipath is
concerned it is just another write request.
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Tested-by: NBharata B Rao <bharata.rao@gmail.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>