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

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: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Borislav 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.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>