dm clone: make array 'descs' static

Don't populate the read-only array descs on the stack but instead it
static and add extra const. Also makes the object code smaller by 66
bytes:

Before:
   text    data     bss     dec     hex filename
  42382   11140     512   54034    d312 ./drivers/md/dm-clone-target.o

After:
   text    data     bss     dec     hex filename
  42220   11236     512   53968    d2d0 ./drivers/md/dm-clone-target.o

(gcc version 11.2.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>