netfs: Only call folio_start_fscache() one time for each folio

netfs: Only call folio_start_fscache() one time for each folio

If a network filesystem using netfs implements a clamp_length()
function, it can set subrequest lengths smaller than a page size.

When we loop through the folios in netfs_rreq_unlock_folios() to
set any folios to be written back, we need to make sure we only
call folio_start_fscache() once for each folio.

Otherwise, this simple testcase:

  mount -o fsc,rsize=1024,wsize=1024 127.0.0.1:/export /mnt/nfs
  dd if=/dev/zero of=/mnt/nfs/file.bin bs=4096 count=1
  1+0 records in
  1+0 records out
  4096 bytes (4.1 kB, 4.0 KiB) copied, 0.0126359 s, 324 kB/s
  echo 3 > /proc/sys/vm/drop_caches
  cat /mnt/nfs/file.bin > /dev/null

will trigger an oops similar to the following:

  page dumped because: VM_BUG_ON_FOLIO(folio_test_private_2(folio))
  ------------[ cut here ]------------
  kernel BUG at include/linux/netfs.h:44!
  ...
  CPU: 5 PID: 134 Comm: kworker/u16:5 Kdump: loaded Not tainted 6.4.0-rc5
  ...
  RIP: 0010:netfs_rreq_unlock_folios+0x68e/0x730 [netfs]
  ...
  Call Trace:
    netfs_rreq_assess+0x497/0x660 [netfs]
    netfs_subreq_terminated+0x32b/0x610 [netfs]
    nfs_netfs_read_completion+0x14e/0x1a0 [nfs]
    nfs_read_completion+0x2f9/0x330 [nfs]
    rpc_free_task+0x72/0xa0 [sunrpc]
    rpc_async_release+0x46/0x70 [sunrpc]
    process_one_work+0x3bd/0x710
    worker_thread+0x89/0x610
    kthread+0x181/0x1c0
    ret_from_fork+0x29/0x50

Fixes: 3d3c95046742 ("netfs: Provide readahead and readpage netfs helpers"
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2210612
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20230608214137.856006-1-dwysocha@redhat.com/
Link: https://lore.kernel.org/r/20230915185704.1082982-1-dwysocha@redhat.com/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>