null_blk: Fix two sparse warnings

Fix the following sparse warnings:

drivers/block/null_blk/main.c:1243:35: warning: incorrect type in return expression (different base types)
drivers/block/null_blk/main.c:1243:35:    expected int
drivers/block/null_blk/main.c:1243:35:    got restricted blk_status_t
drivers/block/null_blk/main.c:1291:30: warning: incorrect type in return expression (different base types)
drivers/block/null_blk/main.c:1291:30:    expected restricted blk_status_t
drivers/block/null_blk/main.c:1291:30:    got int

Cc: Christoph Hellwig <hch@lst.de>
Cc: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510201816.24921-1-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -1218,7 +1218,7 @@ static int null_transfer(struct nullb *nullb, struct page *page,
 	return err;
 }
 
-static int null_handle_rq(struct nullb_cmd *cmd)
+static blk_status_t null_handle_rq(struct nullb_cmd *cmd)
 {
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
 	struct nullb *nullb = cmd->nq->dev->nullb;

drivers/block/null_blk/trace.h

@@ -36,7 +36,12 @@ TRACE_EVENT(nullb_zone_op,
 	    TP_ARGS(cmd, zone_no, zone_cond),
 	    TP_STRUCT__entry(
 		__array(char, disk, DISK_NAME_LEN)
-		__field(enum req_op, op)
+		/*
+		 * __field() uses is_signed_type(). is_signed_type() does not
+		 * support bitwise types. Use __field_struct() instead because
+		 * it does not use is_signed_type().
+		 */
+		__field_struct(enum req_op, op)
 		__field(unsigned int, zone_no)
 		__field(unsigned int, zone_cond)
 	    ),