[CIFS] remove some redundant argument checks

This patch does kmalloc + memset conversion to kzalloc and removes some
redundant argument checks.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>