manipulator: restrict resizes to requested resizes

Example why this matters: if a disk partition is not trying to be
resized, and it's actual size is not an even multiple of what align_up
supports, then it could get a unrequested non-zero size change.
And if there is no supporting gap for that change, then we can get a
size Exception despite not having asked for a resize.

subiquity/common/filesystem/manipulator.py

@@ -170,7 +170,7 @@ class FilesystemManipulator:
                   disk, spec, partition, gap)
 
         if partition is not None:
-            if 'size' in spec:
+            if 'size' in spec and spec['size'] != partition.size:
                 trailing, gap_size = \
                      gaps.movable_trailing_partitions_and_gap_size(partition)
                 new_size = align_up(spec['size'])