RDMA/rdmavt: Delete unnecessary NULL check

There is no need to check 'rdi->qp_dev' for NULL. The field 'qp_dev'
is created in rvt_register_device() which will fail if the 'qp_dev'
allocation fails in rvt_driver_qp_init(). Overwise this pointer
doesn't changed and passed to rvt_qp_exit() by the next step.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 0acb0cc7

 ("IB/rdmavt: Initialize and teardown of qpn table")
Signed-off-by: Natalia Petrova <n.petrova@fintech.ru>
Link: https://lore.kernel.org/r/20230303124408.16685-1-n.petrova@fintech.ru
Signed-off-by: Leon Romanovsky <leon@kernel.org>