powerpc: Fix transactional FP/VMX/VSX unavailable handlers

Currently, if a process starts a transaction and then takes an
exception because the FPU, VMX or VSX unit is unavailable to it,
we end up corrupting any FP/VMX/VSX state that was valid before
the interrupt.  For example, if the process starts a transaction
with the FPU available to it but VMX unavailable, and then does
a VMX instruction inside the transaction, the FP state gets
corrupted.

Loading up the desired state generally involves doing a reclaim
and a recheckpoint.  To avoid corrupting already-valid state, we have
to be careful not to reload that state from the thread_struct
between the reclaim and the recheckpoint (since the thread_struct
values are stale by now), and we have to reload that state from
the transact_fp/vr arrays after the recheckpoint to get back the
current transactional values saved there by the reclaim.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>