btrfs: remove local generation variable from read_block_for_search()

It's redundant to have the 'gen' variable since we already have the same
value in the local btrfs_tree_parent_check structure. So remove it and
instead use the structure's field.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>