disklabel.py: Fix OpenBSD part end when enlarging

When enlarging OpenBSD partitions, round them up to multiples of 16512
which is the default fs blocksize. This is what OpenBSD's disklabel
does when creating partitions.

snf-image-helper/disklabel.py

@@ -825,21 +825,24 @@ class OpenBSDDisklabel(DisklabelBase):
     def enlarge_last_partition(self):
         """Enlarge the last partition to cover up all the free space"""
 
-        part_num = self.get_last_partition_id()
+        last_id = self.get_last_partition_id()
+        start = self.ptable.getpoffset(last_id)
+        size = self.ptable.getpsize(last_id)
 
-        end = self.ptable.getpsize(part_num) + self.ptable.getpoffset(part_num)
+        assert size > 0, "No partition found"
 
-        assert end > 0, "No partition found"
-
-        if self.ptable.part[part_num].fstype == 1:  # Swap partition.
+        if self.ptable.part[last_id].fstype == 1:  # Swap partition.
             warn("Last partition is swap. Not enlarging it.")
             return
 
-        if end > (self.bend - 1024):
+        # Round the partition to 16512B. This is the default bsize in OpenBSD
+        new_size = (self.bend - start) // 32 * 32
+
+        if size >= new_size:
+            warn("No partition resize performed, partition is large enough.")
             return
 
-        self.ptable.setpsize(
-            part_num, self.bend - self.ptable.getpoffset(part_num) - 1024)
+        self.ptable.setpsize(last_id, new_size)
 
         self.field['checksum'] = self.compute_checksum()