btrfs: make extent_map_end() argument const

The extent map pointer argument for extent_map_end() can be const as we
are not modifyng anything in the extent map. So make it const, as it will
allow further changes to callers that have a const extent map pointer.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.h

@@ -64,7 +64,7 @@ static inline int extent_map_in_tree(const struct extent_map *em)
 	return !RB_EMPTY_NODE(&em->rb_node);
 }
 
-static inline u64 extent_map_end(struct extent_map *em)
+static inline u64 extent_map_end(const struct extent_map *em)
 {
 	if (em->start + em->len < em->start)
 		return (u64)-1;