block: use proper return value from bio_failfast()

kernel test robot complains about a type mismatch:

   block/blk-merge.c:984:42: sparse:     expected restricted blk_opf_t const [usertype] ff
   block/blk-merge.c:984:42: sparse:     got unsigned int
   block/blk-merge.c:1010:42: sparse: sparse: incorrect type in initializer (different base types) @@     expected restricted blk_opf_t const [usertype] ff @@     got unsigned int @@
   block/blk-merge.c:1010:42: sparse:     expected restricted blk_opf_t const [usertype] ff
   block/blk-merge.c:1010:42: sparse:     got unsigned int

because bio_failfast() is return an unsigned int rather than the
appropriate blk_opt_f type. Fix it up.

Fixes: 3ce6a115 ("block: sync mixed merged request's failfast with 1st bio's")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202302170743.GXypM9Rt-lkp@intel.com/Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -757,7 +757,7 @@ void blk_rq_set_mixed_merge(struct request *rq)
 	rq->rq_flags |= RQF_MIXED_MERGE;
 }
 
-static inline unsigned int bio_failfast(const struct bio *bio)
+static inline blk_opf_t bio_failfast(const struct bio *bio)
 {
 	if (bio->bi_opf & REQ_RAHEAD)
 		return REQ_FAILFAST_MASK;