bsd-user/mmap.c: Always zero MAP_ANONYMOUS memory in mmap_frag()

bsd-user/mmap.c: Always zero MAP_ANONYMOUS memory in mmap_frag()

Similar to the equivalent linux-user commit e6deac9cf99

When mapping MAP_ANONYMOUS memory fragments, still need notice about to
set it zero, or it will cause issues.

Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>