f2fs: use PGP_LOCK to check its truncation

Previously, after trylock_page is succeeded, it doesn't check its mapping.
In order to fix that, we can just give PGP_LOCK to pagecache_get_page.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 1a33de9..ade221c 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -1202,13 +1202,10 @@
 	if (!inode)
 		return;
 
-	page = pagecache_get_page(inode->i_mapping, 0, FGP_NOWAIT, 0);
+	page = pagecache_get_page(inode->i_mapping, 0, FGP_LOCK|FGP_NOWAIT, 0);
 	if (!page)
 		goto iput_out;
 
-	if (!trylock_page(page))
-		goto release_out;
-
 	if (!PageUptodate(page))
 		goto page_out;
 
@@ -1223,9 +1220,7 @@
 	else
 		set_page_dirty(page);
 page_out:
-	unlock_page(page);
-release_out:
-	f2fs_put_page(page, 0);
+	f2fs_put_page(page, 1);
 iput_out:
 	iput(inode);
 }