open.2, rename.2: Refer to tmpfs rather than shmem

Skimming open(2), I was surprised not to see tmpfs mentioned as a
filesystem supported by O_TMPFILE.

If I'm understanding correctly (I'm very possibly not!), tmpfs is
a filesystem built on shmem, so I think it's more correct (and
probably much more widely understandable) to refer to tmpfs here.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>