cifs: Fix signature miscalculation

Fix the calculation of packet signatures by adding the offset into a page
in the read or write data payload when hashing the pages from it.

Fixes: 39bc5820 ("cifs: Add a function to Hash the contents of an iterator")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Tom Talpey <tom@talpey.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.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/smb/client/cifsencrypt.c

@@ -129,7 +129,7 @@ static ssize_t cifs_shash_xarray(const struct iov_iter *iter, ssize_t maxsize,
 			for (j = foffset / PAGE_SIZE; j < npages; j++) {
 				len = min_t(size_t, maxsize, PAGE_SIZE - offset);
 				p = kmap_local_page(folio_page(folio, j));
-				ret = crypto_shash_update(shash, p, len);
+				ret = crypto_shash_update(shash, p + offset, len);
 				kunmap_local(p);
 				if (ret < 0)
 					return ret;