shm: Check `fcntl` return value

Olivier Fourdan requested to merge ofourdan/xserver:xshm-fcntl into master

Adding seals to the memfd_create()'ed file descriptor could potentially fail.

Make sure to test the return value of the call to fcntl to place the seal.

Fixes: f6753c11 - shm: Use memfd_create when possible Signed-off-by: Olivier Fourdan

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