btrfs: constify and cleanup variables in comparators

Comparators just read the data and thus get const parameters. This
should be also preserved by the local variables, update all comparators
passed to sort or bsearch.

Cleanups:

- unnecessary casts are dropped
- btrfs_cmp_device_free_bytes is cleaned up to follow the common pattern
  and 'inline' is dropped as the function address is taken

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-log.c

@@ -4191,7 +4191,7 @@ static noinline int copy_items(struct btrfs_trans_handle *trans,
 static int extent_cmp(void *priv, const struct list_head *a,
 		      const struct list_head *b)
 {
-	struct extent_map *em1, *em2;
+	const struct extent_map *em1, *em2;
 
 	em1 = list_entry(a, struct extent_map, list);
 	em2 = list_entry(b, struct extent_map, list);