[IA64] kexec: Unpin the correct VHPT TR in ia64_do_tlb_purge

[IA64] kexec: Unpin the correct VHPT TR in ia64_do_tlb_purge

GET_VA_VCPU_VHPT_MADDR(r2,r3) does not give the value of the VHPT
pinned into the TLB. I believe that this is because
current is changed between pinning and calling play_dead,
though I am not sure of the exact scemantics.

In any case, by recording the pinned value in a percpu variable,
and unpinning this value, the TR entry is removed and all is well.

Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Simon Horman <horms@verge.net.au>