btrfs: initialize fs_info::csum_size earlier in open_ctree

User reported that btrfs-progs misc-tests/028-superblock-recover fails:

[TEST/misc] 028-superblock-recover
unexpected success: mounted fs with corrupted superblock
test failed for case 028-superblock-recover

The test case expects that a broken image with bad superblock will be
rejected to be mounted. However, the test image just passed csum check
of superblock and was successfully mounted.

Commit 55fc29bed8dd ("btrfs: use cached value of fs_info::csum_size
everywhere") replaces all calls to btrfs_super_csum_size by
fs_info::csum_size. The calls include the place where fs_info->csum_size
is not initialized. So btrfs_check_super_csum() passes because memcmp()
with len 0 always returns 0.

Fix it by caching csum size in btrfs_fs_info::csum_size once we know the
csum type in superblock is valid in open_ctree().

Link: https://github.com/kdave/btrfs-progs/issues/250
Fixes: 55fc29bed8dd ("btrfs: use cached value of fs_info::csum_size everywhere")
Signed-off-by: Su Yue <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/disk-io.c

···
 		goto fail_alloc;
 	}
 
+	fs_info->csum_size = btrfs_super_csum_size(disk_super);
+
 	ret = btrfs_init_csum_hash(fs_info, csum_type);
 	if (ret) {
 		err = ret;
···
 	fs_info->nodesize = nodesize;
 	fs_info->sectorsize = sectorsize;
 	fs_info->sectorsize_bits = ilog2(sectorsize);
-	fs_info->csum_size = btrfs_super_csum_size(disk_super);
 	fs_info->csums_per_leaf = BTRFS_MAX_ITEM_SIZE(fs_info) / fs_info->csum_size;
 	fs_info->stripesize = stripesize;