modpost: split new_symbol() to symbol allocation and hash table addition

new_symbol() does two things; allocate a new symbol and register it
to the hash table.

Using a separate function for each is easier to understand.

Replace new_symbol() with hash_add_symbol(). Remove the second parameter
of alloc_symbol().

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