RDMA/rxe: Fix panic when calling kmem_cache_create()

RDMA/rxe: Fix panic when calling kmem_cache_create()

To avoid the following kernel panic when calling kmem_cache_create() with
a NULL pointer from pool_cache(), Block the rxe_param_set_add() from
running if the rdma_rxe module is not initialized.

 BUG: unable to handle kernel NULL pointer dereference at 000000000000000b
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP NOPTI
 CPU: 4 PID: 8512 Comm: modprobe Kdump: loaded Not tainted 4.18.0-231.el8.x86_64 #1
 Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 10/02/2018
 RIP: 0010:kmem_cache_alloc+0xd1/0x1b0
 Code: 8b 57 18 45 8b 77 1c 48 8b 5c 24 30 0f 1f 44 00 00 5b 48 89 e8 5d 41 5c 41 5d 41 5e 41 5f c3 81 e3 00 00 10 00 75 0e 4d 89 fe <41> f6 47 0b 04 0f 84 6c ff ff ff 4c 89 ff e8 cc da 01 00 49 89 c6
 RSP: 0018:ffffa2b8c773f9d0 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000005
 RDX: 0000000000000004 RSI: 00000000006080c0 RDI: 0000000000000000
 RBP: ffff8ea0a8634fd0 R08: ffffa2b8c773f988 R09: 00000000006000c0
 R10: 0000000000000000 R11: 0000000000000230 R12: 00000000006080c0
 R13: ffffffffc0a97fc8 R14: 0000000000000000 R15: 0000000000000000
 FS:  00007f9138ed9740(0000) GS:ffff8ea4ae800000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 000000000000000b CR3: 000000046d59a000 CR4: 00000000003406e0
 Call Trace:
  rxe_alloc+0xc8/0x160 [rdma_rxe]
  rxe_get_dma_mr+0x25/0xb0 [rdma_rxe]
  __ib_alloc_pd+0xcb/0x160 [ib_core]
  ib_mad_init_device+0x296/0x8b0 [ib_core]
  add_client_context+0x11a/0x160 [ib_core]
  enable_device_and_get+0xdc/0x1d0 [ib_core]
  ib_register_device+0x572/0x6b0 [ib_core]
  ? crypto_create_tfm+0x32/0xe0
  ? crypto_create_tfm+0x7a/0xe0
  ? crypto_alloc_tfm+0x58/0xf0
  rxe_register_device+0x19d/0x1c0 [rdma_rxe]
  rxe_net_add+0x3d/0x70 [rdma_rxe]
  ? dev_get_by_name_rcu+0x73/0x90
  rxe_param_set_add+0xaf/0xc0 [rdma_rxe]
  parse_args+0x179/0x370
  ? ref_module+0x1b0/0x1b0
  load_module+0x135e/0x17e0
  ? ref_module+0x1b0/0x1b0
  ? __do_sys_init_module+0x13b/0x180
  __do_sys_init_module+0x13b/0x180
  do_syscall_64+0x5b/0x1a0
  entry_SYSCALL_64_after_hwframe+0x65/0xca
 RIP: 0033:0x7f9137ed296e

This can be triggered if a user tries to use the 'module option' which is
not actually a real module option but some idiotic (and thankfully no
obsolete) sysfs interface.

Fixes: 8700e3e7c485 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20200825151725.254046-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>