cifs: have cifsFileInfo hold a reference to a tlink rather than tcon pointer

cifsFileInfo needs a pointer to a tcon, but it doesn't currently hold a
reference to it. Change it to keep a pointer to a tcon_link instead and
hold a reference to it.

That will keep the tcon from being freed until the file is closed.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 898d2a5..b2fd075 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -595,7 +595,8 @@
 	    ((arg == F_WRLCK) &&
 		(CIFS_I(inode)->clientCanCacheAll)))
 		return generic_setlease(file, arg, lease);
-	else if (cfile->tcon->local_lease && !CIFS_I(inode)->clientCanCacheRead)
+	else if (tlink_tcon(cfile->tlink)->local_lease &&
+		 !CIFS_I(inode)->clientCanCacheRead)
 		/* If the server claims to support oplock on this
 		   file, then we still need to check oplock even
 		   if the local_lease mount option is set, but there