svcrdma: Add fair queuing for Send Queue access

When the Send Queue fills, multiple threads may wait for SQ slots.
The previous implementation had no ordering guarantee, allowing
starvation when one thread repeatedly acquires slots while others
wait indefinitely.

Introduce a ticket-based fair queuing system. Each waiter takes a
ticket number and is served in FIFO order. This ensures forward
progress for all waiters when SQ capacity is constrained.

The implementation has two phases:
1. Fast path: attempt to reserve SQ slots without waiting
2. Slow path: take a ticket, wait for turn, then wait for slots

The ticket system adds two atomic counters to the transport:
- sc_sq_ticket_head: next ticket to issue
- sc_sq_ticket_tail: ticket currently being served

A dedicated wait queue (sc_sq_ticket_wait) handles ticket
ordering, separate from sc_send_wait which handles SQ capacity.
This separation ensures that send completions (the high-frequency
wake source) wake only the current ticket holder rather than all
queued waiters. Ticket handoff wakes only the ticket wait queue,
and each ticket holder that exits via connection close propagates
the wake to the next waiter in line.

When a waiter successfully reserves slots, it advances the tail
counter and wakes the next waiter. This creates an orderly handoff
that prevents starvation while maintaining good throughput on the
fast path when contention is low.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

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