linux-user: fix target_mprotect/target_munmap error return values
    
    target_mprotect/target_munmap return value goes through get_errno at the
    call site, thus the functions must either set errno to host error code
    and return -1 or return negative guest error code. Do the latter.
    
    Cc: qemu-stable@nongnu.org
    Cc: Riku Voipio <riku.voipio@iki.fi>
    Cc: Laurent Vivier <laurent@vivier.eu>
    Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
    Reviewed-by: Laurent Vivier <laurent@vivier.eu>
    Message-Id: <20180228221609.11265-8-jcmvbkbc@gmail.com>
    Signed-off-by: Laurent Vivier <laurent@vivier.eu>
    (cherry picked from commit 78cf339039c325b336442f1d7f3ccc531b22c4a0)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>