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.

[v-songbaohua@oppo.com: figure out the cause and correct the commit message]
Link: https://lkml.kernel.org/r/20240618065647.21791-1-21cnbao@gmail.com
Fixes: 2282679 ("mm: submit multipage write for SWP_FS_OPS swap-space")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Closes: https://lore.kernel.org/linux-mm/20240617053201.GA16852@lst.de/
Reviewed-by: Martin Wege <martin.l.wege@gmail.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Steve French <sfrench@samba.org>
Cc: Trond Myklebust <trondmy@kernel.org>
Cc: Chuanhua Han <hanchuanhua@oppo.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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