btrfs: dump extra info if one free space cache has more bitmaps than it should

There is an internal report on hitting the following ASSERT() in
recalculate_thresholds():

 	ASSERT(ctl->total_bitmaps <= max_bitmaps);

Above @max_bitmaps is calculated using the following variables:

- bytes_per_bg
  8 * 4096 * 4096 (128M) for x86_64/x86.

- block_group->length
  The length of the block group.

@max_bitmaps is the rounded up value of block_group->length / 128M.

Normally one free space cache should not have more bitmaps than above
value, but when it happens the ASSERT() can be triggered if
CONFIG_BTRFS_ASSERT is also enabled.

But the ASSERT() itself won't provide enough info to know which is going
wrong.
Is the bg too small thus it only allows one bitmap?
Or is there something else wrong?

So although I haven't found extra reports or crash dump to do further
investigation, add the extra info to make it more helpful to debug.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
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/free-space-cache.c

@@ -693,6 +693,12 @@ static void recalculate_thresholds(struct btrfs_free_space_ctl *ctl)
 
 	max_bitmaps = max_t(u64, max_bitmaps, 1);
 
+	if (ctl->total_bitmaps > max_bitmaps)
+		btrfs_err(block_group->fs_info,
+"invalid free space control: bg start=%llu len=%llu total_bitmaps=%u unit=%u max_bitmaps=%llu bytes_per_bg=%llu",
+			  block_group->start, block_group->length,
+			  ctl->total_bitmaps, ctl->unit, max_bitmaps,
+			  bytes_per_bg);
 	ASSERT(ctl->total_bitmaps <= max_bitmaps);
 
 	/*