btrfs: add macros to facilitate printing of keys

There's a lot of places where we need to print a key, and it's tiresome
to type the format specifier, typically "(%llu %u %llu)", as well as
passing 3 arguments to a prink family function (key->objectid, key->type,
key->offset).

So add a couple macros for this just like we have for csum values in
btrfs_inode.h (CSUM_FMT and CSUM_FMT_VALUE).

This also ensures that we consistently print a key in the same format,
always as "(%llu %llu %llu)", which is the most common format we use, but
we have a few variations such as "[%llu %llu %llu]" for no good reason.

This patch introduces the macros while the next one makes use of it.
This is to ease backports of future patches, since then we can backport
this patch which is simple and short and then backport those future
patches, as the next patch in the series that makes use of these new
macros is quite large and may have some dependencies.

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

fs/btrfs/fs.h

@@ -74,6 +74,9 @@ struct btrfs_space_info;
 #define BTRFS_SUPER_INFO_SIZE			4096
 static_assert(sizeof(struct btrfs_super_block) == BTRFS_SUPER_INFO_SIZE);
 
+#define BTRFS_KEY_FMT			"(%llu %u %llu)"
+#define BTRFS_KEY_FMT_VALUE(key)	(key)->objectid, (key)->type, (key)->offset
+
 /*
  * Number of metadata items necessary for an unlink operation:
  *