btrfs: relocation: constify parameters where possible

[ Upstream commit ab7c8bbf ]

Lots of the functions in relocation.c don't change pointer parameters
but lack the annotations. Add them and reformat according to current
coding style if needed.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: db7e68b5 ("btrfs: drop the backref cache during relocation if we commit")
Signed-off-by: Sasha Levin <sashal@kernel.org>