btrfs: inline checksum name and driver definitions

There's an unnecessary indirection in the checksum definition table,
pointer and the string itself. The strings are short and the overall
size of one entry is now 24 bytes.

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