RDMA/core: Clean up a variable name in ib_create_srq_user()

"&srq->pd->usecnt" and "&pd->usecnt" are different names for the same
reference count.  Use "&pd->usecnt" consistently for both the increment
and decrement.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YyxFe3Pm0uzRuBkQ@kili


Signed-off-by: Leon Romanovsky <leon@kernel.org>