This patch converts qla2xxx target code to use generic percpu_ida
tag allocation provided by target-core, thus removing the original
kmem_cache_zalloc() for each struct qla_tgt_cmd descriptor in the
incoming ATIO packet fast-path.

This includes the conversion of qlt_handle_cmd_for_atio() to perform
qla_tgt_sess lookup before dispatching a command descriptor into
qla_tgt_wq process context, along with handling the case where no
active session exists, and subsequently kicking off a seperate
process context for qlt_create_sess_from_atio() to create a new one.

It also includes moving tag allocation into generic code within
qlt_get_tag(), so that the same logic can be shared between
qlt_handle_cmd_for_atio() + qlt_create_sess_from_atio() contexts.
Also, __qlt_do_work() has been made generic between both normal
process context in qlt_do_work() + qlt_create_sess_from_atio().

Next, update qlt_free_cmd() to release the percpu-ida tags, and
drop the now-unused global qla_tgt_cmd_cachep.

Finally in tcm_qla2xxx code, tcm_qla2xxx_check_initiator_node_acl()
has been updated to use transport_init_session_tags() along with a
hardcoded TCM_QLA2XXX_DEFAULT_TAGS=2088 as the number of qla_tgt_cmd
descriptors to pre-allocate per qla_tgt_sess instance.

(Use ha->fw_xcb_count if available to calculate num_tags, and
 also factor in extra pad tags - Quinn)

Cc: Saurav Kashyap <saurav.kashyap@qlogic.com>
Cc: Quinn Tran <quinn.tran@qlogic.com>
Cc: Giridhar Malavali <giridhar.malavali@qlogic.com>
Cc: Chad Dupuis <chad.dupuis@qlogic.com>
Cc: Roland Dreier <roland@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>