Fix for use after free in the LinuxKPI.

Background:
The same VM object might be shared by multiple processes and the
mm_struct is usually freed when a process exits.

Grab a reference on the mm_struct while the vmap is in the
linux_vma_head list in case the first process which inserted a VM
object has exited.

Tested by: kwm @
MFC after: 1 week
Sponsored by: Mellanox Technologies