btrfs: remove bogus root search condition in sample_block_group_extent_item()

btrfs: remove bogus root search condition in sample_block_group_extent_item()

There's no need to pass the maximum between the block group's start offset
and BTRFS_SUPER_INFO_OFFSET (64K) since we can't have any block groups
allocated in the first megabyte, as that's reserved space. Furthermore,
even if we could, the correct thing to do was to pass the block group's
start offset anyway - and that's precisely what we do for block groups
that happen to contain superblock mirror (the range for the super block
is never marked as free and it's marked as dirty in the
fs_info->excluded_extents io tree).

So simplify this and get rid of that maximum expression.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>