util: fix an invalid memset() of alloc_bitmap()

Current alloc_bitmap() calls memset() with an address new_bmap + old_size. The
expression produces an illegally large address because new_bmap is declared as a
pointer of unsigned long, but the code seems to assume new_bmap is declared as
char *. This patch corrects the invalid address caliculation.

Cc: Gerald Richter <richter@ecos.de>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>