export kernel_write(), convert open-coded instances

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ecryptfs/read_write.c b/fs/ecryptfs/read_write.c
index b2a34a1..6a16053 100644
--- a/fs/ecryptfs/read_write.c
+++ b/fs/ecryptfs/read_write.c
@@ -40,16 +40,12 @@
 			 loff_t offset, size_t size)
 {
 	struct file *lower_file;
-	mm_segment_t fs_save;
 	ssize_t rc;
 
 	lower_file = ecryptfs_inode_to_private(ecryptfs_inode)->lower_file;
 	if (!lower_file)
 		return -EIO;
-	fs_save = get_fs();
-	set_fs(get_ds());
-	rc = vfs_write(lower_file, data, size, &offset);
-	set_fs(fs_save);
+	rc = kernel_write(lower_file, data, size, offset);
 	mark_inode_dirty_sync(ecryptfs_inode);
 	return rc;
 }