powerpc/bpf: Simplify bpf_to_ppc() and adopt it for powerpc64

Convert bpf_to_ppc() to a macro to help simplify its usage since
codegen_context is available in all places it is used. Adopt it also for
powerpc64 for uniformity and get rid of the global b2p structure.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/09f0540ce3e0cd4120b5b33993b5e73b6ef9e979.1644834730.git.naveen.n.rao@linux.vnet.ibm.com