csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum

Preparation for the change of calling conventions; right now all
callers pass 0 as initial sum.  Passing 0xffffffff instead yields
the values comparable mod 0xffff and guarantees that 0 will not
be returned on success.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -1452,7 +1452,7 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
 		int err = 0;
 		next = csum_and_copy_from_user(v.iov_base,
 					       (to += v.iov_len) - v.iov_len,
-					       v.iov_len, 0, &err);
+					       v.iov_len, ~0U, &err);
 		if (!err) {
 			sum = csum_block_add(sum, next, off);
 			off += v.iov_len;
@@ -1494,7 +1494,7 @@ bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum,
 		int err = 0;
 		next = csum_and_copy_from_user(v.iov_base,
 					       (to += v.iov_len) - v.iov_len,
-					       v.iov_len, 0, &err);
+					       v.iov_len, ~0U, &err);
 		if (err)
 			return false;
 		sum = csum_block_add(sum, next, off);
@@ -1540,7 +1540,7 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump,
 		int err = 0;
 		next = csum_and_copy_to_user((from += v.iov_len) - v.iov_len,
 					     v.iov_base,
-					     v.iov_len, 0, &err);
+					     v.iov_len, ~0U, &err);
 		if (!err) {
 			sum = csum_block_add(sum, next, off);
 			off += v.iov_len;