libfdisk: fix last free sector detection if partition size specified

libfdisk: fix last free sector detection if partition size specified

We need to skip useless gaps between partition if the gap is no large
enough for a new partition. Unfortunately, the current code checks
size of the gap, but does not care for location of the gap -- this is
good enough for dialog driven partitioning, but it's pretty bad if
start of the partition is explicitly specified (e.g. sfdisk).

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1860461
Signed-off-by: Karel Zak <kzak@redhat.com>