modpost: make sym_add_exported() always allocate a new symbol

Currently, sym_add_exported() does not allocate a symbol if the same
name symbol already exists in the hash table.

This does not reflect the real use cases. You can let an external
module override the in-tree one. In this case, the external module
will export the same name symbols as the in-tree one. However,
modpost simply ignores those symbols, then Module.symvers for the
external module loses its symbols.

sym_add_exported() should allocate a new symbol.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>