btrfs: add a btrfs_csum_type_size helper

This is needed in btrfs-progs for the tools that convert the checksum
types for file systems and a few other things.  We don't have it in the
kernel as we just want to get the size for the super blocks type.
However I don't want to have to manually add this every time we sync
ctree.c into btrfs-progs, so add the helper in the kernel with a note so
it doesn't get removed by a later cleanup.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -150,13 +150,19 @@ static inline void copy_leaf_items(const struct extent_buffer *dst,
 			      nr_items * sizeof(struct btrfs_item));
 }
 
+/* This exists for btrfs-progs usages. */
+u16 btrfs_csum_type_size(u16 type)
+{
+	return btrfs_csums[type].size;
+}
+
 int btrfs_super_csum_size(const struct btrfs_super_block *s)
 {
 	u16 t = btrfs_super_csum_type(s);
 	/*
 	 * csum type is validated at mount time
 	 */
-	return btrfs_csums[t].size;
+	return btrfs_csum_type_size(t);
 }
 
 const char *btrfs_super_csum_name(u16 csum_type)

fs/btrfs/ctree.h

@@ -701,6 +701,7 @@ static inline bool btrfs_is_data_reloc_root(const struct btrfs_root *root)
 	return root->root_key.objectid == BTRFS_DATA_RELOC_TREE_OBJECTID;
 }
 
+u16 btrfs_csum_type_size(u16 type);
 int btrfs_super_csum_size(const struct btrfs_super_block *s);
 const char *btrfs_super_csum_name(u16 csum_type);
 const char *btrfs_super_csum_driver(u16 csum_type);