lib/digsig: remove unnecessary memset

kzalloc() has already been initialized to full 0 space, there is no need
to use memset() to initialize again.

Link: https://lkml.kernel.org/r/20250811082739.378284-1-liaoyuanhong@vivo.com


Signed-off-by: Liao Yuanhong <liaoyuanhong@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>