dm-stripe: small code cleanup

This commit doesn't fix any bug, it is just code cleanup. Use the
function format_dev_t instead of sprintf, because format_dev_t does the
same thing.

Remove the useless memset call.

An unsigned integer can take at most 10 digits, so extend the array size
to 22. (note that because the range of minor and major numbers is limited,
the size 16 could not be exceeded, thus this function couldn't write
beyond string end)

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>