cifs: Fix cifs_limit_bvec_subset() to correctly check the maxmimum size

Fix cifs_limit_bvec_subset() so that it limits the span to the maximum
specified and won't return with a size greater than max_size.

Fixes: d08089f6 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Cc: stable@vger.kernel.org # 6.3
Reported-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <smfrench@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Tom Talpey <tom@talpey.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/file.c

@@ -3353,9 +3353,10 @@ static size_t cifs_limit_bvec_subset(const struct iov_iter *iter, size_t max_siz
 	while (n && ix < nbv) {
 		len = min3(n, bvecs[ix].bv_len - skip, max_size);
 		span += len;
+		max_size -= len;
 		nsegs++;
 		ix++;
-		if (span >= max_size || nsegs >= max_segs)
+		if (max_size == 0 || nsegs >= max_segs)
 			break;
 		skip = 0;
 		n -= len;