f2fs: use block layer helper for show_bio_op macro

Adjust the f2fs tracing code to use newly introduced block layer
function blk_op_str() which converts the REQ_OP_XXX into the string
XXX.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>