linux-user: Tolerate CONFIG_LSM_MMAP_MIN_ADDR

linux-user: Tolerate CONFIG_LSM_MMAP_MIN_ADDR

Running qemu-i386 on a system running with SELinux in enforcing mode
(more precisely: s390x trixie container on Fedora 40) fails with:

    qemu-i386: tests/tcg/i386-linux-user/sigreturn-sigmask: Unable to find a guest_base to satisfy all guest address mapping requirements
      00000000-ffffffff

The reason is that main() determines mmap_min_addr from
/proc/sys/vm/mmap_min_addr, but SELinux additionally defines
CONFIG_LSM_MMAP_MIN_ADDR, which is normally larger: 32K or 64K, but,
in general, can be anything. There is no portable way to query its
value: /boot/config, /proc/config and /proc/config.gz are distro- and
environment-specific.

Once the identity map fails, the magnitude of guest_base does not
matter, so fix by starting the search from 1M or 1G.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2598
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20241023002558.34589-1-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit fb7f3572b111ffb6c2dd2c7f6c5b4dc57dd8a3f5)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>