btrfs: use ncopies from btrfs_raid_array in btrfs_num_copies()

For all non-RAID56 profiles, we can use btrfs_raid_array[].ncopies
directly, only for RAID5 and RAID6 we need some extra handling as
there's no table value for that.

For RAID10 there's a change from sub_stripes to ncopies. The values are
the same but semantically we want to use number of copies, as this is
what btrfs_num_copies does.

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>