linux-user: init_guest_space: Don't try to align if we'll reject it

linux-user: init_guest_space: Don't try to align if we'll reject it

If the ensure-alignment code gets triggered, then the
"if (host_start && real_start != current_start)" check will always trigger,
so save 2 syscalls and put that check first.

Note that we can't just switch to using MAP_FIXED for that check, because
then we couldn't differentiate between a failure because "there isn't
enough space" and "there isn't enough space *here*".

Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
Message-Id: <20171228180814.9749-9-lukeshu@lukeshu.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>