initramfs: fix initramfs to work with hardlinked init

Change cb6ff20 ("NOMMU: Support XIP on
initramfs") seems to have broken booting from initramfs with /sbin/init
being a hardlink.

It seems like the logic required for XIP on nommu, i.e.  ftruncate to
reported cpio header file size (body_len) is broken for hardlinks, which
have a reported size of 0, and the truncate thus nukes the contents of the
file (in my case busybox), making boot impossible and ending with runaway
loop modprobe binfmt-0000 - and of course 0000 is not a valid binary
format.

My fix is to only call ftruncate if size is non-zero which fixes things
for me, but I'm not certain whether this will break XIP for those files on
nommu systems, although I would guess not.

Signed-off-by: Randy Robertson <[email protected]>
Acked-by: David Howells <[email protected]>
Acked-by: Paul Mundt <[email protected]>
Acked-by: H. Peter Anvin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

init/initramfs.c

@@ -310,7 +310,8 @@ static int __init do_name(void)
 			if (wfd >= 0) {
 				sys_fchown(wfd, uid, gid);
 				sys_fchmod(wfd, mode);
-				sys_ftruncate(wfd, body_len);
+				if (body_len)
+					sys_ftruncate(wfd, body_len);
 				vcollected = kstrdup(collected, GFP_KERNEL);
 				state = CopyFile;
 			}