block: bio: ensure newly added bio flags don't override BVEC_POOL_IDX

With the introduction of BIO_NO_PAGE_REF we've used up all available bits
in bio::bi_flags.

Convert the defines of the flags to an enum and add a BUILD_BUG_ON() call
to make sure no-one adds a new one and thus overrides the BVEC_POOL_IDX
causing crashes.

Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/bio.c

@@ -2218,6 +2218,9 @@ static int __init init_bio(void)
 	bio_slab_nr = 0;
 	bio_slabs = kcalloc(bio_slab_max, sizeof(struct bio_slab),
 			    GFP_KERNEL);
+
+	BUILD_BUG_ON(BIO_FLAG_LAST > BVEC_POOL_OFFSET);
+
 	if (!bio_slabs)
 		panic("bio: can't allocate bios\n");