staging: rdma: Use kcalloc instead of kzalloc

Use kcalloc rather than kzalloc when multiplied by size to prevent
integer overflows. This change also makes code nicer to read.

Signed-off-by: Janani Ravichandran <janani.rvchndrn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>