powerpc: Add ppc_inst_as_u64()

powerpc: Add ppc_inst_as_u64()

The code patching code wants to get the value of a struct ppc_inst as
a u64 when the instruction is prefixed, so we can pass the u64 down to
__put_user_asm() and write it with a single store.

The optprobes code wants to load a struct ppc_inst as an immediate
into a register so it is useful to have it as a u64 to use the
existing helper function.

Currently this is a bit awkward because the value differs based on the
CPU endianness, so add a helper to do the conversion.

This fixes the usage in arch_prepare_optimized_kprobe() which was
previously incorrect on big endian.

Fixes: 650b55b707fd ("powerpc: Add prefixed instructions to instruction data type")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Jordan Niethe <jniethe5@gmail.com>
Link: https://lore.kernel.org/r/20200526072630.2487363-1-mpe@ellerman.id.au