netfs: Fix a KMSAN uninit-value error in netfs_clear_buffer

Use folioq_count instead of folioq_nr_slots to fix a KMSAN uninit-value
error in netfs_clear_buffer

Signed-off-by: Chang Yu <marcus.yu.56@gmail.com>
Link: https://lore.kernel.org/r/ZvuXWC2bYpvQsWgS@gmail.com


Fixes: cd0277ed ("netfs: Use new folio_queue data type and iterator instead of xarray iter")
Acked-by: David Howells <dhowells@redhat.com>
Reported-by:  <syzbot+921873345a95f4dae7e9@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=921873345a95f4dae7e9


Signed-off-by: Christian Brauner <brauner@kernel.org>