ovl: fix wrong flags check in FS_IOC_FS[SG]ETXATTR ioctls

The ioctl argument was parsed as the wrong type.

Fixes: b21d9c43

 ("ovl: support the FS_IOC_FS[SG]ETXATTR ioctls")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>