Remove easily user-triggerable BUG from generic_setlease

This can be trivially triggered from userspace by passing in something unexpected.

    kernel BUG at fs/locks.c:1468!
    invalid opcode: 0000 [#1] SMP
    RIP: 0010:generic_setlease+0xc2/0x100
    Call Trace:
      __vfs_setlease+0x35/0x40
      fcntl_setlease+0x76/0x150
      sys_fcntl+0x1c6/0x810
      system_call_fastpath+0x1a/0x1f
Signed-off-by: NDave Jones <davej@redhat.com>
Cc: stable@kernel.org # 3.2+
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/locks.c

@@ -1465,7 +1465,7 @@ int generic_setlease(struct file *filp, long arg, struct file_lock **flp)
 	case F_WRLCK:
 		return generic_add_lease(filp, arg, flp);
 	default:
-		BUG();
+		return -EINVAL;
 	}
 }
 EXPORT_SYMBOL(generic_setlease);