unix_connectat's fdescfs cases call mount_fdescfs(), which skipped on
ENODEV. nmount(2) never returns ENODEV: vfs_donmount() remaps the
ENODEV from a failed fdescfs module load to EINVAL with errmsg
"Invalid fstype", so the skip never fired and the cases failed on
kernels without fdescfs.
Use ATF_REQUIRE_KERNEL_MODULE("fdescfs") instead, and drop the dead
ENODEV branch. modfind(2) sees loaded and static modules, not
unloaded .ko files, so a host with fdescfs.ko on disk but not loaded
will now skip rather than having nmount(2) implicitly kldload it.
Upstream CI keeps fdescfs mounted via tests/ci/tools/ci.conf, so it
still runs them.
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