btrfs: move mapping of block for discard to its caller

There's a simple forwarded call based on the operation that would better
fit the caller btrfs_map_block that's until now a trivial wrapper.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>