rds: Remove redundant assignment to nr_sig

Variable nr_sig is being assigned a value however the assignment is
never read, so this redundant assignment can be removed.

Cleans up the following clang-analyzer warning:

net/rds/ib_send.c:297:2: warning: Value stored to 'nr_sig' is never read
[clang-analyzer-deadcode.DeadStores].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>