btrfs: unexport extent_map_block_end()

The helper extent_map_block_end() is currently not used anywhere outside
extent_map.c, so move into from extent_map.h into extent_map.c. While at
it, also make the extent map pointer argument as const.

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