mkfs: simplify the configured sector sizes setting in validate_sectorsize

mkfs: simplify the configured sector sizes setting in validate_sectorsize

There are two places that set the configured sector sizes in
validate_sectorsize, actually we can simplify them and combine into one
if statement. Use the default value structure to set the topology sectors
when probing fails.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>