NFS: Fix typo in nomigration mount option

The option was incorrectly masking off all other options.
Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Cc: stable@vger.kernel.org #3.7
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/super.c

@@ -1332,7 +1332,7 @@ static int nfs_parse_mount_options(char *raw,
 			mnt->options |= NFS_OPTION_MIGRATION;
 			break;
 		case Opt_nomigration:
-			mnt->options &= NFS_OPTION_MIGRATION;
+			mnt->options &= ~NFS_OPTION_MIGRATION;
 			break;
 
 		/*