dm raid: add reshaping support to the target

Add bool functions rs_is_recovering and rs_is_reshaping()
to test for ongoing recovery/reshaping respectively in order
to reject respective requests on ongoing ones.

Remove ctr array size check, because ti->len and array
sectors will differ during disk addition/removal reshape.

Use __is_raid10_near() rather than type string compare.

Introduce rs_check_reshape() and rs_start_reshape(),
use the former in the ctr to reject bogus rehsape requests
and the latter in preresume to actually start a reshape.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>