btrfs: force v2 space cache usage for subpage mount

[BUG]
For a 4K sector sized btrfs with v1 cache enabled and only mounted on
systems with 4K page size, if it's mounted on subpage (64K page size)
systems, it can cause the following warning on v1 space cache:

 BTRFS error (device dm-1): csum mismatch on free space cache
 BTRFS warning (device dm-1): failed to load free space cache for block group 84082688, rebuilding it now

Although not a big deal, as kernel can rebuild it without problem, such
warning will bother end users, especially if they want to switch the
same btrfs seamlessly between different page sized systems.

[CAUSE]
V1 free space cache is still using fixed PAGE_SIZE for various bitmap,
like BITS_PER_BITMAP.

Such hard-coded PAGE_SIZE usage will cause various mismatch, from v1
cache size to checksum.

Thus kernel will always reject v1 cache with a different PAGE_SIZE with
csum mismatch.

[FIX]
Although we should fix v1 cache, it's already going to be marked
deprecated soon.

And we have v2 cache based on metadata (which is already fully subpage
compatible), and it has almost everything superior than v1 cache.

So just force subpage mount to use v2 cache on mount.

Reported-by: Matt Corallo <blnxfsl@bluematt.me>
CC: stable@vger.kernel.org # 5.15+
Link: https://lore.kernel.org/linux-btrfs/61aa27d1-30fc-c1a9-f0f4-9df544395ec3@bluematt.me/
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3657,6 +3657,17 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	if (sectorsize < PAGE_SIZE) {
 		struct btrfs_subpage_info *subpage_info;
 
+		/*
+		 * V1 space cache has some hardcoded PAGE_SIZE usage, and is
+		 * going to be deprecated.
+		 *
+		 * Force to use v2 cache for subpage case.
+		 */
+		btrfs_clear_opt(fs_info->mount_opt, SPACE_CACHE);
+		btrfs_set_and_info(fs_info, FREE_SPACE_TREE,
+			"forcing free space tree for sector size %u with page size %lu",
+			sectorsize, PAGE_SIZE);
+
 		btrfs_warn(fs_info,
 		"read-write for sector size %u with page size %lu is experimental",
 			   sectorsize, PAGE_SIZE);