linux-user: fix target_mprotect/target_munmap error return values

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>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>