ubifs: fix page_count in ->ubifs_migrate_page()

[zhangjun]

Because the PagePrivate() in UBIFS is different meanings,
alloc_cma() will fail when one dirty page cache located in
the type of MIGRATE_CMA

If not adjust the 'extra_count' for dirty page,
ubifs_migrate_page() -> migrate_page_move_mapping() will
always return -EAGAIN for:
	expected_count += page_has_private(page)
This causes the migration to fail until the page cache is cleaned

In general, PagePrivate() indicates that buff_head is already bound
to this page, and at the same time page_count() will also increase.
But UBIFS set private flag when the cache is dirty, and page_count()
not increase.
Therefore, the expected_count of UBIFS is different from the general
case.

Signed-off-by: zhangjun <openzhangj@xxxxxxxxx>
---
 fs/ubifs/file.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 1b78f2e..2136a5c 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1480,8 +1480,15 @@ static int ubifs_migrate_page(struct address_space *mapping,
 		struct page *newpage, struct page *page, enum migrate_mode mode)
 {
 	int rc;
+	int extra_count;
 
-	rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
+	/*
+	 * UBIFS is using PagePrivate() which can have different meanings across
+	 * filesystems. So here adjusting the 'extra_count' make it work.
+	 */
+	extra_count = 0 - page_has_private(page);
+	rc = migrate_page_move_mapping(mapping, newpage,
+			page, NULL, mode, extra_count);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
 
-- 
2.7.4