checkpatch: avoid sscanf test duplicated messages

Change a test of $dstat to $line to avoid possibly emitting the sscanf
warning multiple times.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>