FOPEN_CACHE_DIR was added in Linux 4.19:

	https://git.kernel.org/linus/6433b8998a

Provide InodeLink/InodeEmbed which implements linking machinery.

This is unlikely to be useful, but makes the API more uniform.

Use this to simplify loopbackNode.

This is an implementation detail, but it saves an indirection

syscall.Close is not safe to run twice. Reused fds cause random things
to break.

Test Lseek together with FOPEN_DIRECT_IO, and document this use-case
in directio_test.go