mm/vmscan.c: support removing arbitrary sized pages from mapping

mm/vmscan.c: support removing arbitrary sized pages from mapping

__remove_mapping() assumes that pages can only be either base pages or
HPAGE_PMD_SIZE.  Ask the page what size it is.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: William Kucharski <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>