jbd2: use offset_in_page() instead of manual calculation

This is a port to jbd2 of a patch which Namhyung Kim <namhyung@gmail.com>
originally made to fs/jbd.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -774,7 +774,7 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
 		J_EXPECT_JH(jh, buffer_uptodate(jh2bh(jh)),
 			    "Possible IO failure.\n");
 		page = jh2bh(jh)->b_page;
-		offset = ((unsigned long) jh2bh(jh)->b_data) & ~PAGE_MASK;
+		offset = offset_in_page(jh2bh(jh)->b_data);
 		source = kmap_atomic(page, KM_USER0);
 		/* Fire data frozen trigger just before we copy the data */
 		jbd2_buffer_frozen_trigger(jh, source + offset,