shm: Check `fcntl` return value

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 <ofourdan@redhat.com>
3 jobs for xshm-fcntl in 7 minutes and 12 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed debian-testing #1119288

00:00:44

 
  Build And Test
passed autotools-build-and-test #1119289

00:06:28

passed meson-build-and-test #1119290

00:04:04