ALSA: core: fix buffer overflow in test_format_fill_silence()

KASAN caught a buffer overflow with the hardcoded 2048 byte buffer
size, when 2080 bytes are written to it:

 BUG: KASAN: slab-out-of-bounds in snd_pcm_format_set_silence+0x3bc/0x3e4
 Write of size 8 at addr ffff0000c8149800 by task kunit_try_catch/1297

 CPU: 0 PID: 1297 Comm: kunit_try_catch Tainted: G N 6.8.0-rc4-next-20240216 #1
 Hardware name: linux,dummy-virt (DT)
 Call trace:
  kasan_report+0x78/0xc0
  __asan_report_store_n_noabort+0x1c/0x28
  snd_pcm_format_set_silence+0x3bc/0x3e4
  _test_fill_silence+0xdc/0x298
  test_format_fill_silence+0x110/0x228
  kunit_try_run_case+0x144/0x3bc
  kunit_generic_run_threadfn_adapter+0x50/0x94
  kthread+0x330/0x3e8
  ret_from_fork+0x10/0x20

 Allocated by task 1297:
  __kmalloc+0x17c/0x2f0
  kunit_kmalloc_array+0x2c/0x78
  test_format_fill_silence+0xcc/0x228
  kunit_try_run_case+0x144/0x3bc
  kunit_generic_run_threadfn_adapter+0x50/0x94
  kthread+0x330/0x3e8
  ret_from_fork+0x10/0x20

Replace the incorrect size with the correct length of 260 64-bit samples.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Suggested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Fixes: 3e39acf5

 ("ALSA: core: Add sound core KUnit test")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240217104311.3749655-1-arnd@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>