dm raid: add write_mostly parameter

Add the write_mostly parameter to RAID1 dm-raid tables.

This allows the user to set the WriteMostly flag on a RAID1 device that
should normally be avoided for read I/O.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>