[PATCH] btrfs: uapi: fix stale BTRFS_SYSTEM_CHUNK_ARRAY_SIZE comment

[Sun YangKai <sunk67188@gmail.com>]

The comment claims room for "14 chunks with 3 stripes each", which
was correct when the structs were smaller.  Since then three additions
grew each entry:

  commit e17cade25ff8 ("Btrfs: Add chunk uuids and update multi-device back references")
    added btrfs_chunk.length (+8) and stripe.dev_uuid (+16)

  commit 321aecc65671 ("Btrfs: Add RAID10 support")
    added btrfs_chunk.sub_stripes (+2)

A 3-stripe entry now takes:

  sizeof(btrfs_disk_key) + btrfs_chunk_item_size(3)
    = 17 + 80 + 32 * (3 - 1)
    = 161 bytes

2048 / 161 ≈ 12.7, so "14" is no longer achievable.  Update to 12
and add the explicit calculation so it does not rot again.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
---
 include/uapi/linux/btrfs_tree.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index cc3b9f7dccaf..78404abc4ba2 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -504,7 +504,11 @@ struct btrfs_header {
 
 /*
  * This is a very generous portion of the super block, giving us room to
- * translate 14 chunks with 3 stripes each.
+ * translate 12 chunks with 3 stripes each.
+ *
+ * Each entry takes sizeof(btrfs_disk_key) + btrfs_chunk_item_size(n),
+ * which for a 3-stripe chunk is 17 + 80 + 32 * (3 - 1) = 161 bytes.
+ * 2048 / 161 ≈ 12.7, so at most 12 entries fit.
  */
 #define BTRFS_SYSTEM_CHUNK_ARRAY_SIZE 2048
 
-- 
2.54.0