enic: add MBOX core send and receive for admin channel

Implement the mailbox protocol engine used for PF-VF communication
over the admin channel.

The send path (enic_mbox_send_msg) builds a message with a common
header, DMA-maps it, posts a single WQ descriptor with the
destination vnic ID encoded in the VLAN tag field, and polls
the WQ CQ for completion.

The total message length is computed as a size_t, and the payload is
bounded before the send lock is taken: a payload larger than the admin
buffer minus the header is rejected with -EINVAL.  This keeps the length
sum from wrapping and stops the on-the-wire u16 length from overflowing
or the DMA buffer from being overrun.

MBOX sends are gated by enic->mbox_send_disabled: enic_mbox_send_msg()
returns early while it is set.  It is set at the very start of both
enic_admin_channel_open() and enic_admin_channel_close(), and is
cleared in enic_admin_channel_open() only once the admin WQ/RQ/CQ and
interrupt are fully allocated, programmed and enabled.  Keeping it set
for the whole open sequence means an early failure that returns before
the channel is ready (as well as a not-yet-ready or torn-down channel)
leaves sends disabled, so a concurrent sender can never race an MBOX
send against a half-open or freed admin_wq.

The receive path (enic_mbox_recv_handler) is installed as the admin
RQ callback and validates incoming message headers. PF/VF-specific
dispatch will be added in subsequent commits.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Link: https://patch.msgid.link/20260812-enic-sriov-v2-admin-channel-v2-v13-6-b3809e448aba@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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