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.