cifs: call cifs_update_eof with i_lock held

cifs_update_eof has the potential to be racy if multiple threads are
trying to modify it at the same time. Protect modifications of the
server_eof value with the inode->i_lock.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
index d7e39b0..4ff6313 100644
--- a/fs/cifs/cifsglob.h
+++ b/fs/cifs/cifsglob.h
@@ -626,7 +626,7 @@
 	bool delete_pending;		/* DELETE_ON_CLOSE is set */
 	bool invalid_mapping;		/* pagecache is invalid */
 	unsigned long time;		/* jiffies of last update of inode */
-	u64  server_eof;		/* current file size on server */
+	u64  server_eof;		/* current file size on server -- protected by i_lock */
 	u64  uniqueid;			/* server inode number */
 	u64  createtime;		/* creation time on server */
 #ifdef CONFIG_CIFS_FSCACHE