ceph: allow sync_read/write return partial successed size of read/write.

For sync_read/write, it may do multi stripe operations.If one of those
met erro, we return the former successed size rather than a error value.
There is a exception for write-operation met -EOLDSNAPC.If this occur,we
retry the whole write again.
Signed-off-by: NJianpeng Ma <majianpeng@gmail.com>

fs/ceph/file.c

@@ -374,7 +374,7 @@ static int striped_read(struct inode *inode,
 			goto more;
 	}
 
-	if (ret >= 0) {
+	if (read > 0) {
 		ret = read;
 		/* did we bounce off eof? */
 		if (pos + left > inode->i_size)
@@ -612,6 +612,8 @@ static ssize_t ceph_sync_write(struct file *file, const char __user *data,
 		if (check_caps)
 			ceph_check_caps(ceph_inode(inode), CHECK_CAPS_AUTHONLY,
 					NULL);
+	} else if (ret != -EOLDSNAPC && written > 0) {
+		ret = written;
 	}
 	return ret;
 }