bcachefs: allocate sb_read_scratch with __get_free_page

kmalloc allocations aren't guranteed alignment for io
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super-io.c

@@ -652,7 +652,7 @@ static void read_back_super(struct bch_fs *c, struct bch_dev *ca)
 
 	bio_reset(bio, ca->disk_sb.bdev, REQ_OP_READ|REQ_SYNC|REQ_META);
 	bio->bi_iter.bi_sector	= le64_to_cpu(sb->layout.sb_offset[0]);
-	bio->bi_iter.bi_size	= 4096;
+	bio->bi_iter.bi_size	= PAGE_SIZE;
 	bio->bi_end_io		= write_super_endio;
 	bio->bi_private		= ca;
 	bch2_bio_map(bio, ca->sb_read_scratch);

fs/bcachefs/super.c

@@ -919,7 +919,7 @@ static void bch2_dev_free(struct bch_dev *ca)
 	free_percpu(ca->io_done);
 	bioset_exit(&ca->replica_set);
 	bch2_dev_buckets_free(ca);
-	kfree(ca->sb_read_scratch);
+	free_page((unsigned long) ca->sb_read_scratch);
 
 	bch2_time_stats_exit(&ca->io_latency[WRITE]);
 	bch2_time_stats_exit(&ca->io_latency[READ]);
@@ -1030,7 +1030,7 @@ static struct bch_dev *__bch2_dev_alloc(struct bch_fs *c,
 			    0, GFP_KERNEL) ||
 	    percpu_ref_init(&ca->io_ref, bch2_dev_io_ref_complete,
 			    PERCPU_REF_INIT_DEAD, GFP_KERNEL) ||
-	    !(ca->sb_read_scratch = kmalloc(4096, GFP_KERNEL)) ||
+	    !(ca->sb_read_scratch = (void *) __get_free_page(GFP_KERNEL)) ||
 	    bch2_dev_buckets_alloc(c, ca) ||
 	    bioset_init(&ca->replica_set, 4,
 			offsetof(struct bch_write_bio, bio), 0) ||