compat: update comment of compat statfs syscalls

The commit 7ed1ee61 ("Take statfs variants to fs/statfs.c")
separates out statfs syscalls from fs/open.c. Thus the comment
should be changed also.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>