Fix malloc_increase is not correctly calculated

[peterzhu2118]

Commit 123eeb1 added incremental GC which moved resetting malloc_increase, oldmalloc_increase to before marking. However, during_minor_gc is not set until gc_marks_start. So the value will be from the last GC run, rather than the current one. Before the incremental GC commit, this code was in gc_before_sweep which ran before sweep (after marking) so the value during_minor_gc was correct.

If we run the following script:

arr = []

# Ensure that all objects are promoted to old by triggering GC
3.times do
  GC.start
end

before = GC.stat(:malloc_increase_bytes)
before_old = GC.stat(:oldmalloc_increase_bytes)

# Expand arr so that it reallocates
100.times do
  arr << 0
end

after = GC.stat(:malloc_increase_bytes)
after_old = GC.stat(:oldmalloc_increase_bytes)

# Minor GC
GC.start(full_mark: false)

after_gc = GC.stat(:malloc_increase_bytes)
after_gc_old = GC.stat(:oldmalloc_increase_bytes)

puts before, after, after_gc
puts before_old, after_old, after_gc_old

Before this patch we get this output:

1064
2096
0
1064
2096
0

After this patch we get this output:

272
1304
0
272
1304
1320

A minor GC should not reset oldmalloc_increase_bytes.