x86-pasid-2022-03-21

 - Simplify the PASID handling to allocate the PASID once, associate it to
   the mm of a process and free it on mm_exit(). The previous attempt of
   refcounted PASIDs and dynamic alloc()/free() turned out to be error
   prone and too complex. The PASID space is 20bits, so the case of
   resource exhaustion is a pure academic concern.

 - Populate the PASID MSR on demand via #GP to avoid racy updates via IPIs.

 - Reenable ENQCMD and let objtool check for the forbidden usage of ENQCMD
   in the kernel.

 - Update the documentation for Shared Virtual Addressing accordingly.