linux-user/mmap.c: Use end instead of real_end in target_mmap

The fragment must effectively be mapped only to "end" not to "real_end"
(which is a host page aligned address, and thus this is not a fragment).
It is consistent with what it is done in the case of one single page.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>