btrfs: print-tree: print key types as human readable strings

Looking at a leaf dump from the kernel's print-tree implementation is not
so friendly to analyze since key types are printed as numbers. Improve on
this by printing key types as strings that are a diminutive of the macro
names for key types, just like we do in btrfs-progs.

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