Before registering a new mm_struct in the LinuxKPI check if other
tasks in the belonging procedure already have a valid mm_struct and
reference that instead.

The mm_struct in the LinuxKPI should be shared among all tasks
belonging to the same procedure. This has to do with with the mmap_sem
semaphore which should serialize all VM operations inside a given
procedure. Linux based drivers depend on this behaviour.

MFC after: 1 week
Sponsored by: Mellanox Technologies