powerpc/bpf/32: No need to zeroise r4 when not doing tail call

r4 is cleared at function entry and used as tail call count.

But when the function does not perform tail call, r4 is
ignored, so no need to clear it.

Replace it by a NOP in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9c5440b2b6d90a78600257433ac499b5c5101fbb.1675245773.git.christophe.leroy@csgroup.eu