repart: fix free area calculation

Like fdisk_get_last_lba(), fdisk_partition_get_end() return the last
sector in the partition.

Fixes #28225.

(cherry picked from commit d2eb1f8)
(cherry picked from commit 2299ca1)

Author: yuwata

src/partition/repart.c

@@ -1751,6 +1751,8 @@ static int determine_current_padding(
                 return log_error_errno(SYNTHETIC_ERRNO(EIO), "Partition has no end!");
 
         offset = fdisk_partition_get_end(p);
+        assert(offset < UINT64_MAX);
+        offset++; /* The end is one sector before the next partition or padding. */
         assert(offset < UINT64_MAX / secsz);
         offset *= secsz;