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/cifssmb.c b/fs/cifs/cifssmb.c
index 5ec0b90..b63bf5f 100644
--- a/fs/cifs/cifssmb.c
+++ b/fs/cifs/cifssmb.c
@@ -2044,7 +2044,9 @@
 	int i = 0;
 
 	if (wdata->result == 0) {
+		spin_lock(&inode->i_lock);
 		cifs_update_eof(CIFS_I(inode), wdata->offset, wdata->bytes);
+		spin_unlock(&inode->i_lock);
 		cifs_stats_bytes_written(tlink_tcon(wdata->cfile->tlink),
 					 wdata->bytes);
 	} else if (wdata->sync_mode == WB_SYNC_ALL && wdata->result == -EAGAIN)