nvmet: fix a memory leak in nvmet_auth_set_key

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

Fixes: db1312dd9548 ("nvmet: implement basic In-Band Authentication")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>