nvme_tcp: add psk_identity to nvme_tcp_ctrlr structure

PSK identity will be generated starting with the next patch.
This string could be stored on stack inside
nvme_tcp_qpair_connect_sock() function, but it would expire
later on, so any attempts to access it would fail.
To avoid heap allocation and later freeing that memory,
add new field "psk_identity" to nvme_tcp_ctrlr structure
and store PSK identity string there.

Change-Id: Id1e6527747ccd53859ae89b3813712123853f0b9
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/17295


Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>