libbpf: keep FD_CLOEXEC flag when dup()'ing FD

Merge Request Required Information

Summary of Changes

Backporting a fix for a libbpf bug which causes various systemd generators trigger { map_read map_write } denials.

Since the issue is a blocker and the fix is not a part of the stable tree (i.e. won't probably get to 6.10), I'm posting this separately rather than waiting for regular rebases.

Approved Development Ticket

JIRA: https://issues.redhat.com/browse/RHEL-44675

Signed-off-by: Viktor Malik vmalik@redhat.com