mm/shmem: make find_get_pages_range() work for huge page

find_get_pages_range() and find_get_pages_range_tag() already correctly
increment reference count on head when seeing compound page, but they
may still use page index from tail.  Page index from tail is always
zero, so these functions don't work on huge shmem.  This hasn't been a
problem because, AFAIK, nobody calls these functions on (huge) shmem.
Fix them anyway just in case.

Link: http://lkml.kernel.org/r/20190110030838.84446-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: "Darrick J . Wong" <darrick.wong@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Yu Zhao

mm/filemap.c

@@ -1789,7 +1789,7 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
 
 		pages[ret] = page;
 		if (++ret == nr_pages) {
-			*start = page->index + 1;
+			*start = xas.xa_index + 1;
 			goto out;
 		}
 		continue;
@@ -1927,7 +1927,7 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
 
 		pages[ret] = page;
 		if (++ret == nr_pages) {
-			*index = page->index + 1;
+			*index = xas.xa_index + 1;
 			goto out;
 		}
 		continue;