nfs: drop the incorrect assertion in nfs_swap_rw()

Since commit 2282679 ("mm: submit multipage write for SWP_FS_OPS
swap-space"), we can plug multiple pages then unplug them all together. 
That means iov_iter_count(iter) could be way bigger than PAGE_SIZE, it
actually equals the size of iov_iter_npages(iter, INT_MAX).

Note this issue has nothing to do with large folios as we don't support
THP_SWPOUT to non-block devices.

[[email protected]: figure out the cause and correct the commit message]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 2282679 ("mm: submit multipage write for SWP_FS_OPS swap-space")
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Barry Song <[email protected]>
Closes: https://lore.kernel.org/linux-mm/[email protected]/
Reviewed-by: Martin Wege <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Anna Schumaker <[email protected]>
Cc: Steve French <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Chuanhua Han <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Chris Li <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Christoph Hellwig

fs/nfs/direct.c

@@ -141,8 +141,6 @@ int nfs_swap_rw(struct kiocb *iocb, struct iov_iter *iter)
 {
 	ssize_t ret;
 
-	VM_BUG_ON(iov_iter_count(iter) != PAGE_SIZE);
-
 	if (iov_iter_rw(iter) == READ)
 		ret = nfs_file_direct_read(iocb, iter, true);
 	else