RDMA/rxe: Fix rxe_qp_cleanup()

rxe_qp_cleanup() can sleep so it must be run in thread context and
not in atomic context. This patch avoids that the following bug is
triggered:

Kernel BUG at 00000000560033f3 [verbose debug info unavailable]
BUG: sleeping function called from invalid context at net/core/sock.c:2761
in_atomic(): 1, irqs_disabled(): 0, pid: 7, name: ksoftirqd/0
INFO: lockdep is turned off.
Preemption disabled at:
[<00000000b6e69628>] __do_softirq+0x4e/0x540
CPU: 0 PID: 7 Comm: ksoftirqd/0 Not tainted 4.15.0-rc7-dbg+ #4
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 dump_stack+0x85/0xbf
 ___might_sleep+0x177/0x260
 lock_sock_nested+0x1d/0x90
 inet_shutdown+0x2e/0xd0
 rxe_qp_cleanup+0x107/0x140 [rdma_rxe]
 rxe_elem_release+0x18/0x80 [rdma_rxe]
 rxe_requester+0x1cf/0x11b0 [rdma_rxe]
 rxe_do_task+0x78/0xf0 [rdma_rxe]
 tasklet_action+0x99/0x270
 __do_softirq+0xc0/0x540
 run_ksoftirqd+0x1c/0x70
 smpboot_thread_fn+0x1be/0x270
 kthread+0x117/0x130
 ret_from_fork+0x24/0x30

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Moni Shoua <monis@mellanox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Doug Ledford <dledford@redhat.com>