dm: fix spelling errors

Fixed some confusing spelling errors that were currently identified,
the details are as follows:

-in the code comments:
        dm-cache-target.c: 1371:        exclussive      ==> exclusive
        dm-raid.c: 2522:                repective       ==> respective

Signed-off-by: Shen Lichuan <shenlichuan@vivo.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>