Otherwise we fail like this on my Fedora 38 box:
=== RUN TestOverlay
DetectQuirks: tmpfs detected, no extended attributes except acls will work.
root_test.go:379: No user xattrs! overlay mount will likely fail.
15:15:57.957960 Unimplemented opcode OPCODE-51
root_test.go:398: mount: /tmp/gocryptfs-test-parent-0/3652394902/TestOverlay.2374697046.mnt/merged: wrong fs type, bad option, bad superblock on overlay, missing codepage or helper program, or other error.
dmesg(1) may have more information after failed mount system call.
root_test.go:399: exit status 32
--- FAIL: TestOverlay (0.04s)
FAIL
Also fix the messed-up DetectQuirks bit test.