block: add a BUILD_BUG_ON() for adding more bio flags than we have space

We have BIO_FLAG_LAST in the enum for bio specific flags, but it's
not used to check that we're not exceeding the size of them. Add
such a check.
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1792,6 +1792,8 @@ static int __init init_bio(void)
 {
 	int i;
 
+	BUILD_BUG_ON(BIO_FLAG_LAST > 8 * sizeof_field(struct bio, bi_flags));
+
 	bio_integrity_init();
 
 	for (i = 0; i < ARRAY_SIZE(bvec_slabs); i++) {