genksyms: refactor the return points in the for-loop in __add_symbol()

free_list() must be called before returning from this for-loop.

Swap 'break' and the combination of free_list() and 'return'.

This reduces the code and minimizes the risk of introducing memory
leaks in future changes.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>