mmap: fail if flags contain both MAP_SHARED and MAP_PRIVATE

Currently, we only check if neither MAP_PRIVATE nor MAP_SHARED were
passed to mmap, however, if it was called with both flags, then EINVAL
should be returned.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>