CacheFiles: Fixup renamed filenames in comments in internal.h

Fix up renamed filenames in comments in fs/cachefiles/internal.h.

Originally, the files were all called cf-xxx.c, but they got renamed to
just xxx.c.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>