mm/page_ext: remove rollback for untouched mem_section in online_page_ext

mm/page_ext: remove rollback for untouched mem_section in online_page_ext

If init_section_page_ext failed, we only need rollback for mem_section
before failed mem_section.  Make rollback end point to failed mem_section
to remove unnecessary rollback.

As pfn += PAGES_PER_SECTION will be executed even if init_section_page_ext
failed.  So pfn points to mem_section after failed mem_section.  Subtract
one mem_section from pfn to get failed mem_section.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kemeng Shi <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>