Fix mremap so that only accounted memory is unaccounted if the mapping is
expandable but vma_merge() fails.

Link: https://lkml.kernel.org/r/20230830004549.16131-1-anthony.yznaga@oracle.com
Fixes: fdbef614 ("mm/mremap: don't account pages in vma_to_resize()")
Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Acked-by: Brian Geffon <bgeffon@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>