porting: document block device freeze and thaw changes

Link: https://lore.kernel.org/r/20230927-vfs-super-freeze-v1-7-ecc36d9ab4d9@kernel.org
Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-9-599c19f4faac@kernel.orgReviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/porting.rst

@@ -1061,3 +1061,15 @@ export_operations ->encode_fh() no longer has a default implementation to
 encode FILEID_INO32_GEN* file handles.
 Filesystems that used the default implementation may use the generic helper
 generic_encode_ino32_fh() explicitly.
+
+---
+
+**recommended**
+
+Block device freezing and thawing have been moved to holder operations.
+
+Before this change, get_active_super() would only be able to find the
+superblock of the main block device, i.e., the one stored in sb->s_bdev. Block
+device freezing now works for any block device owned by a given superblock, not
+just the main block device. The get_active_super() helper and bd_fsfreeze_sb
+pointer are gone.