RDMA/srpt: Add a check for valid 'mad_agent' pointer

In the Linux kernel, the following vulnerability has been resolved:

RDMA/srpt: Add a check for valid 'mad_agent' pointer

When unregistering MAD agent, srpt module has a non-null check
for 'mad_agent' pointer before invoking ib_unregister_mad_agent().
This check can pass if 'mad_agent' variable holds an error value.
The 'mad_agent' can have an error value for a short window when
srpt_add_one() and srpt_remove_one() is executed simultaneously.

In srpt module, added a valid pointer check for 'sport->mad_agent'
before unregistering MAD agent.

This issue can hit when RoCE driver unregisters ib_device

Stack Trace:
------------
BUG: kernel NULL pointer dereference, address: 000000000000004d
PGD 145003067 P4D 145003067 PUD 2324fe067 PMD 0
Oops: 0002 [#1] PREEMPT SMP NOPTI
CPU: 10 PID: 4459 Comm: kworker/u80:0 Kdump: loaded Tainted: P
Hardware name: Dell Inc. PowerEdge R640/06NR82, BIOS 2.5.4 01/13/2020
Workqueue: bnxt_re bnxt_re_task [bnxt_re]
RIP: 0010:_raw_spin_lock_irqsave+0x19/0x40
Call Trace:
ib_unregister_mad_agent+0x46/0x2f0 [ib_core]
IPv6: ADDRCONF(NETDEV_CHANGE): bond0: link becomes ready
? __schedule+0x20b/0x560
srpt_unregister_mad_agent+0x93/0xd0 [ib_srpt]
srpt_remove_one+0x20/0x150 [ib_srpt]
remove_client_context+0x88/0xd0 [ib_core]
bond0: (slave p2p1): link status definitely up, 100000 Mbps full duplex
disable_device+0x8a/0x160 [ib_core]
bond0: active interface up!
? kernfs_name_hash+0x12/0x80
(NULL device *): Bonding Info Received: rdev: 000000006c0b8247
__ib_unregister_device+0x42/0xb0 [ib_core]
(NULL device *): Master: mode: 4 num_slaves:2
ib_unregister_device+0x22/0x30 [ib_core]
(NULL device *): Slave: id: 105069936 name:p2p1 link:0 state:0
bnxt_re_stopqps_and_ib_uninit+0x83/0x90 [bnxt_re]
bnxt_re_alloc_lag+0x12e/0x4e0 [bnxt_re]