nvmet: fix a memory leak in nvmet_auth_set_key

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

nvmet: fix a memory leak in nvmet_auth_set_key

When changing dhchap secrets we need to release the old
secrets as well.

kmemleak complaint:
--
unreferenced object 0xffff8c7f44ed8180 (size 64):
comm "check", pid 7304, jiffies 4295686133 (age 72034.246s)
hex dump (first 32 bytes):
44 48 48 43 2d 31 3a 30 30 3a 4c 64 4c 4f 64 71 DHHC-1:00:LdLOdq
79 56 69 67 77 48 55 32 6d 5a 59 4c 7a 35 59 38 yVigwHU2mZYLz5Y8
backtrace:
[<00000000b6fc5071>] kstrdup+0x2e/0x60
[<00000000f0f4633f>] 0xffffffffc0e07ee6
[<0000000053006c05>] 0xffffffffc0dff783
[<00000000419ae922>] configfs_write_iter+0xb1/0x120
[<000000008183c424>] vfs_write+0x2be/0x3c0
[<000000009005a2a5>] ksys_write+0x5f/0xe0
[<00000000cd495c89>] do_syscall_64+0x38/0x90
[<00000000f2a84ac5>] entry_SYSCALL_64_after_hwframe+0x63/0xcd