proc: Open anon_dev mapped files with O_RDONLY

Commit a3a10c44 made all map_files opens happen with
O_PATH for speed. However some files (sockets and aio
rings) are not detected by this, since kernel's ->open
callback is not called and we don't get the ENXIO code.

Fix this by opening non-regular files with O_RDONLY.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>