reproducible "panic: share->excl"

[Andrew Gallatin]

Kostik Belousov wrote:

> Essentially, you tried to do the write of the part of the region mmaped
> from the file, to the file. The VOP_WRITE() is called with exclusively
> locked vnode, while fault handler tried to lock the vnode in shared mode
> to page in.

Yes, the operations attempted were rather nonsensical, but should
not have killed the system.  Thank you for looking into it.

Drew