btrfs: use btrfs_chunk_max_errors() to replace tolerance calculation

In __btrfs_map_block() we have an assignment to @max_errors using
nr_parity_stripes().

Although it works for RAID56 it's confusing.  Replace it with
btrfs_chunk_max_errors().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>