LU-6356 tgt: handle sec context requests properly

LU-6356 tgt: handle sec context requests properly

When dealing with security context initialization requests,
no export is associated with the request yet, because these
requests are sent before *_CONNECT requests.
So the normal *_common_slice handlers will not be called,
given there is no reference to the target.

So add a special case for handling of SEC_CTX_INIT, SEC_CTX_INIT_CONT
and SEC_CTX_FINI requests in tgt_handle_request0() and
tgt_request_handle(), in order to directly send the reply.

Also remove tgt_sec_ctx_handlers because they become useless.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Change-Id: Ic0947082c7acc188ebbf2e9cb9ff57e999b5ed3e
Reviewed-on: http://review.whamcloud.com/14041
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>