module: Make is_mapping_symbol() return bool

The return value of is_mapping_symbol() is true or false,
so change its type to reflect that.

Suggested-by: Xi Zhang <zhangxi@kylinos.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>