The two paravirt callbacks .mmu.activate_mm and .mmu.dup_mmap are
sharing the same implementations in all cases: for Xen PV guests they
are pinning the PGD of the new mm_struct, and for all other cases
they are a NOP.
So merge them to a common callback .mmu.enter_mmap (in contrast to the
corresponding already existing .mmu.exit_mmap).
As the first parameter of the old callbacks isn't used, drop it from
the replacement one.
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>