ext4: replace ext4_io_submit->io_op with ->io_wbc

ext4_io_submit_init() takes the pointer to writeback_control to test
its sync_mode and determine between WRITE and WRITE_SYNC and records
the result in ->io_op.  This patch makes it record the pointer
directly and moves the test to ext4_io_submit().

This doesn't cause any noticeable differences now but having
writeback_control available throughout IO submission path will be
depended upon by the planned cgroup writeback support.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index f5e9f04..32071f5 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -187,7 +187,7 @@
 } ext4_io_end_t;
 
 struct ext4_io_submit {
-	int			io_op;
+	struct writeback_control *io_wbc;
 	struct bio		*io_bio;
 	ext4_io_end_t		*io_end;
 	sector_t		io_next_block;