switch generic_write_checks() to iocb and iter

... returning -E... upon error and amount of data left in iter after
(possible) truncation upon success.  Note, that normal case gives
a non-zero (positive) return value, so any tests for != 0 _must_ be
updated.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Conflicts:
	fs/ext4/file.c
diff --git a/fs/udf/file.c b/fs/udf/file.c
index ccab8b7..3de2eda 100644
--- a/fs/udf/file.c
+++ b/fs/udf/file.c
@@ -120,21 +120,15 @@
 	ssize_t retval;
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file_inode(file);
-	size_t count = iov_iter_count(from);
 	struct udf_inode_info *iinfo = UDF_I(inode);
 	int err;
 
 	mutex_lock(&inode->i_mutex);
 
-	retval = generic_write_checks(file, &iocb->ki_pos, &count);
-	if (retval)
+	retval = generic_write_checks(iocb, from);
+	if (retval <= 0)
 		goto out;
 
-	if (count == 0)
-		goto out;
-
-	iov_iter_truncate(from, count);
-
 	down_write(&iinfo->i_data_sem);
 	if (iinfo->i_alloc_type == ICBTAG_FLAG_AD_IN_ICB) {
 		loff_t end = iocb->ki_pos + iov_iter_count(from);