Fix the following sparse warning:

drivers/md/raid10.c:2647:60: sparse: sparse: incorrect type in argument 5 (different base types) @@     expected restricted blk_opf_t [usertype] opf @@     got int rw @@

This patch does not change any functionality since REQ_OP_READ = READ = 0
and since REQ_OP_WRITE = WRITE = 1.

Cc: Rong A Chen <rong.a.chen@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Paul Menzel <pmenzel@molgen.mpg.de>
Fixes: 4ce4c73f ("md/core: Combine two sync_page_io() arguments")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Song Liu <song@kernel.org>