bsd-user/mmap.c: mmap return ENOMEM on overflow

bsd-user/mmap.c: mmap return ENOMEM on overflow

mmap should return ENOMEM on len overflow rather than EINVAL. Return
EINVAL when len == 0 and ENOMEM when the rounded to a page length is 0.
Found by make check-tcg.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>