selftests/powerpc: Add test to check if VSRs are corrupted

When a transaction is aborted, VSR values should rollback to the
checkpointed values before the transaction began. VSRs used elsewhere in
the kernel during a transaction, or while the transaction is suspended
should not affect the checkpointed values.

Prior to the bug fix in commit d31626f7

 ("powerpc: Don't corrupt
transactional state when using FP/VMX in kernel") when VMX was requested
by the kernel the .vr_state (which held the checkpointed state of VSRs
before the transaction) was overwritten with the current state from
outside the transation. Thus if the transaction did not complete, the
VSR values would be "rolled back" to potentially incorrect values.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>