locking/mutex: Fix ww_mutex wait_list operations

Chaitanya, John and Mikhail reported commit 25500ba7e77c ("locking/mutex:
Remove the list_head from struct mutex") wrecked ww_mutex.

Specifically there were 2 issues:

 - __ww_waiter_prev() had the termination condition wrong; it would terminate
   when the previous entry was the first, which results in a truncated
   iteration: W3, W2, (no W1).

 - __mutex_add_waiter(@pos != NULL), as used by __ww_waiter_add() /
   __ww_mutex_add_waiter(); this inserts @waiter before @pos (which is what
   list_add_tail() does). But this should then also update lock->first_waiter.

Much thanks to Prateek for spotting the __mutex_add_waiter() issue!

Fixes: 25500ba7e77c ("locking/mutex: Remove the list_head from struct mutex")
Reported-by: "Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com>
Closes: https://lore.kernel.org/r/af005996-05e9-4336-8450-d14ca652ba5d%40intel.com
Reported-by: John Stultz <jstultz@google.com>
Closes: https://lore.kernel.org/r/CANDhNCq%3Doizzud3hH3oqGzTrcjB8OwGeineJ3mwZuGdDWG8fRQ%40mail.gmail.com
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Closes: https://lore.kernel.org/r/CABXGCsO5fKq2nD9nO8yO1z50ZzgCPWqueNXHANjntaswoOh2Dg@mail.gmail.com
Debugged-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://patch.msgid.link/20260422092335.GH3102924%40noisy.programming.kicks-ass.net

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