x86/RAS/mce_amd_inj: Fix signed wrap around when decrementing index 'i'

Change predecrement compare to post decrement compare to avoid an
unsigned integer wrap-around comparisomn when decrementing in the while
loop.

For example, if the debugfs_create_file() fails when 'i' is zero, the
current situation will predecrement 'i' in the while loop, wrapping 'i' to
the maximum signed integer and cause multiple out of bounds reads on
dfs_fls[i].d as the loop interates to zero.

Also, as Borislav Petkov suggested, return -ENODEV rather than -ENOMEM
on the error condition.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yazen Ghannam <Yazen.Ghannam@amd.com>
Link: http://lkml.kernel.org/r/20160926083152.30848-2-bp@alien8.deSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/ras/mce_amd_inj.c

@@ -464,13 +464,13 @@ static int __init init_mce_inject(void)
 	return 0;
 
 err_dfs_add:
-	while (--i >= 0)
+	while (i-- > 0)
 		debugfs_remove(dfs_fls[i].d);
 
 	debugfs_remove(dfs_inj);
 	dfs_inj = NULL;
 
-	return -ENOMEM;
+	return -ENODEV;
 }
 
 static void __exit exit_mce_inject(void)