lib/digsig.c: use ERR_CAST inlined function instead of ERR_PTR(PTR_ERR(...))

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>