ALSA: seq: avoid stale FIFO cells during resize

In the Linux kernel, the following vulnerability has been resolved:

ALSA: seq: avoid stale FIFO cells during resize

snd_seq_fifo_resize() still needs to publish the replacement pool
before it waits for FIFO users. A blocking snd_seq_read() holds
f->use_lock while it sleeps, so concurrent senders must be able to
queue to the new pool and wake that reader instead of failing against a
closing old pool.

However, snd_seq_fifo_event_in() duplicates an event before it takes
f->lock, and snd_seq_read() can dequeue a cell and later call
snd_seq_fifo_cell_putback() if copy_to_user() or
snd_seq_expand_var_event() fails. If resize swaps f->pool and detaches
oldhead in between, either path can relink an old-pool cell after the
snapshot. That stale cell sits outside the drained oldhead list, keeps
oldpool->counter elevated, and can leave snd_seq_pool_delete() waiting
for the retired pool to drain.

Keep the existing swap-before-wait ordering in snd_seq_fifo_resize(),
but reject stale cells before any FIFO relink. Revalidate event-in cells
under f->lock and retry them against the published replacement pool, and
free stale putback cells instead of linking them back into the FIFO.

The buggy scenario involves two paths, with each column showing the
order within that path:

resize path: relink path:
1. Allocate newpool. 1. Take f->use_lock.
2. Swap f->pool to newpool and 2. Duplicate or dequeue an old-pool
detach oldhead. cell before oldpool closes.
3. Mark oldpool closing and 3. Reach a later relink point after
wait for FIFO users. resize published newpool.
4. Free oldhead and delete 4. Relink the old-pool cell after
oldpool. resize detached oldhead.
5. Drop f->use_lock.

The reproducer reports a resize ioctl blocked in the expected pool
teardown path:

signal: resize iteration=98 target_pool=4 exceeded 250ms
(elapsed=251ms)
diagnostic: resize_tid=651 wchan=snd_seq_pool_done
diagnostic: resize_tid=651 stack=
snd_seq_pool_done+0x5b/0x140
snd_seq_pool_delete+0x7a/0x90
snd_seq_fifo_resize+0x193/0x1e0
snd_seq_ioctl_set_client_pool+0x214/0x260
snd_seq_ioctl+0x119/0x540
__x64_sys_ioctl+0xd1/0x120
do_syscall_64+0xbb/0x2f0
entry_SYSCALL_64_after_hwframe+0x77/0x7f

A second run with larger pools hit the same target path:

signal: resize iteration=32 target_pool=64 exceeded 250ms
(elapsed=251ms)
diagnostic: resize_tid=663 wchan=snd_seq_pool_done
diagnostic: resize_tid=663 stack=
snd_seq_pool_done+0x5b/0x140
snd_seq_pool_delete+0x7a/0x90
snd_seq_fifo_resize+0x193/0x1e0
snd_seq_ioctl_set_client_pool+0x214/0x260
snd_seq_ioctl+0x119/0x540
__x64_sys_ioctl+0xd1/0x120
do_syscall_64+0xbb/0x2f0
entry_SYSCALL_64_after_hwframe+0x77/0x7f