proc: Open anon_dev mapped files with O_RDONLY
proc: Open anon_dev mapped files with O_RDONLY

Commit a3a10c44af 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 <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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