RFC: MFC'ng a change to struct mount

Hi,

Since there seemed to be interest in it, I have MFC'd most of
the changes needed to run nfsd(8) in a vnet jail to stable/13.

However, there is one change that *might* affect the VFS KAPI.

I need to add a pointer called mnt_exjail to struct mount.

There is a field called mnt_pad0. It is 32bits, but is followed
by a field that is a structure.  As such, replacing it with the pointer
does not change the offsets of subsequent fields for amd64.
--> Since mnt_exjail is only used in generic code, this should
     not break the VFS KAPI for amd64, I think?
I will check i386, but since it will be replacing a 32bit field with a
32bit pointer, I suspect it will be ok.

Does anyone think some other architecture will be broken by this
or know an easy way I can check all arches?

Thanks for any input, rick