btrfs: avoid using fixed char array size for tree names

[BUG]
There is a bug report that using the latest trunk GCC 15, btrfs would cause
unterminated-string-initialization warning:

  linux-6.6/fs/btrfs/print-tree.c:29:49: error: initializer-string for array of ‘char’ is too long [-Werror=unterminated-string-initialization]
   29 |         { BTRFS_BLOCK_GROUP_TREE_OBJECTID,      "BLOCK_GROUP_TREE"      },
      |
      ^~~~~~~~~~~~~~~~~~

[CAUSE]
To print tree names we have an array of root_name_map structure, which
uses "char name[16];" to store the name string of a tree.

But the following trees have names exactly at 16 chars length:
- "BLOCK_GROUP_TREE"
- "RAID_STRIPE_TREE"

This means we will have no space for the terminating '\0', and can lead
to unexpected access when printing the name.

[FIX]
Instead of "char name[16];" use "const char *" instead.

Since the name strings are all read-only data, and are all NULL
terminated by default, there is not much need to bother the length at
all.

Reported-by: Sam James <sam@gentoo.org>
Reported-by: Alejandro Colomar <alx@kernel.org>
Fixes: edde81f1ab ("btrfs: add raid stripe tree pretty printer")
Fixes: 9c54e80ddc ("btrfs: add code to support the block group root")
CC: stable@vger.kernel.org # 6.1+
Suggested-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/print-tree.c

@@ -14,7 +14,7 @@
 
 struct root_name_map {
 	u64 id;
-	char name[16];
+	const char *name;
 };
 
 static const struct root_name_map root_map[] = {