fm10k: Fix a potential NULL pointer dereference

Syzkaller report this:

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN PTI
CPU: 0 PID: 4378 Comm: syz-executor.0 Tainted: G         C        5.0.0+ #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
RIP: 0010:__lock_acquire+0x95b/0x3200 kernel/locking/lockdep.c:3573
Code: 00 0f 85 28 1e 00 00 48 81 c4 08 01 00 00 5b 5d 41 5c 41 5d 41 5e 41 5f c3 4c 89 ea 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f 85 cc 24 00 00 49 81 7d 00 e0 de 03 a6 41 bc 00 00
RSP: 0018:ffff8881e3c07a40 EFLAGS: 00010002
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000010 RSI: 0000000000000000 RDI: 0000000000000080
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
R10: ffff8881e3c07d98 R11: ffff8881c7f21f80 R12: 0000000000000001
R13: 0000000000000080 R14: 0000000000000000 R15: 0000000000000001
FS:  00007fce2252e700(0000) GS:ffff8881f2400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fffc7eb0228 CR3: 00000001e5bea002 CR4: 00000000007606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 lock_acquire+0xff/0x2c0 kernel/locking/lockdep.c:4211
 __mutex_lock_common kernel/locking/mutex.c:925 [inline]
 __mutex_lock+0xdf/0x1050 kernel/locking/mutex.c:1072
 drain_workqueue+0x24/0x3f0 kernel/workqueue.c:2934
 destroy_workqueue+0x23/0x630 kernel/workqueue.c:4319
 __do_sys_delete_module kernel/module.c:1018 [inline]
 __se_sys_delete_module kernel/module.c:961 [inline]
 __x64_sys_delete_module+0x30c/0x480 kernel/module.c:961
 do_syscall_64+0x9f/0x450 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x462e99
Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fce2252dc58 EFLAGS: 00000246 ORIG_RAX: 00000000000000b0
RAX: ffffffffffffffda RBX: 000000000073bf00 RCX: 0000000000462e99
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000140
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fce2252e6bc
R13: 00000000004bcca9 R14: 00000000006f6b48 R15: 00000000ffffffff

If alloc_workqueue fails, it should return -ENOMEM, otherwise may
trigger this NULL pointer dereference while unloading drivers.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 0a38c17a21a0 ("fm10k: Remove create_workqueue")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
