netfs: Fix a KMSAN uninit-value error in netfs_clear_buffer

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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Fixes: cd0277ed0c18 ("netfs: Use new folio_queue data type and iterator instead of xarray iter")
Acked-by: David Howells <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=921873345a95f4dae7e9
Signed-off-by: Christian Brauner <[email protected]>