dm log: fix unitialized bio operation flags

Commit e6047149 ("dm: use bio op accessors") switched DM over to
using bio_set_op_attrs() but didn't take care to initialize
lc->io_req.bi_op_flags in dm-log.c:rw_header().  This caused
rw_header()'s call to dm_io() to make bio->bi_op_flags be uninitialized
in dm-io.c:do_region(), which ultimately resulted in a SCSI BUG() in
sd_init_command().

Also, adjust rw_header() and its callers to use REQ_OP_{READ|WRITE}.

Fixes: e6047149 ("dm: use bio op accessors")
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>