btrfs: allocate path on stack in load_block_group_size_class()

btrfs: allocate path on stack in load_block_group_size_class()

Instead of allocating and freeing a path in every iteration of
load_block_group_size_class(), through its helper function
sample_block_group_extent_item(), allocate the path in the former and
pass it to the later. The path is allocated on stack since it's short
and we are in a workqueue context so there's not much stack usage.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
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>