[patch] block_invalidatepage: only release page if the full page was invalidated

[Jeff Moyer <jmoyer@xxxxxxxxxx>]

Prior to commit d47992f86b307 ("mm: change invalidatepage prototype to
accept length"), an offset of 0 meant that the full page was being
invalidated.  After that commit, we need to instead check the length.

Fixes: d47992f86b307 ("mm: change invalidatepage prototype to accept length")
Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

---
Note that I found this via code inspection.  I'm not sure if this causes
issues in the wild.

diff --git a/fs/buffer.c b/fs/buffer.c
index 9a73924db22f..6b98609356ed 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1511,7 +1511,7 @@ void block_invalidatepage(struct page *page, unsigned int offset,
 	 * The get_block cached value has been unconditionally invalidated,
 	 * so real IO is not possible anymore.
 	 */
-	if (offset == 0)
+	if (length == PAGE_SIZE)
 		try_to_release_page(page, 0);
 out:
 	return;