sd: Fix potential out-of-bounds access

commit 984f1733fcee3fbc78d47e26c5096921c5d9946a upstream.

This patch fixes an out-of-bounds error in sd_read_cache_type(), found
by Google's AddressSanitizer tool.  When the loop ends, we know that
"offset" lies beyond the end of the data in the buffer, so no Caching
mode page was found.  In theory it may be present, but the buffer size
is limited to 512 bytes.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>