md: use bio_kmalloc rather than bio_alloc when failure is acceptable.

bio_alloc can never fail (as it uses a mempool) but an block
indefinitely, especially if the caller is holding a reference to a
previously allocated bio.

So these to places which both handle failure and hold multiple bios
should not use bio_alloc, they should use bio_kmalloc.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid1.c

@@ -100,7 +100,7 @@ static void * r1buf_pool_alloc(gfp_t gfp_flags, void *data)
 	 * Allocate bios : 1 for reading, n-1 for writing
 	 */
 	for (j = pi->raid_disks ; j-- ; ) {
-		bio = bio_alloc(gfp_flags, RESYNC_PAGES);
+		bio = bio_kmalloc(gfp_flags, RESYNC_PAGES);
 		if (!bio)
 			goto out_free_bio;
 		r1_bio->bios[j] = bio;

drivers/md/raid10.c

@@ -120,7 +120,7 @@ static void * r10buf_pool_alloc(gfp_t gfp_flags, void *data)
 	 * Allocate bios.
 	 */
 	for (j = nalloc ; j-- ; ) {
-		bio = bio_alloc(gfp_flags, RESYNC_PAGES);
+		bio = bio_kmalloc(gfp_flags, RESYNC_PAGES);
 		if (!bio)
 			goto out_free_bio;
 		r10_bio->devs[j].bio = bio;