powerpc/bpf: Always reallocate BPF_REG_5, BPF_REG_AX and TMP_REG when possible

BPF_REG_5, BPF_REG_AX and TMP_REG are mapped on non volatile registers
because there are not enough volatile registers, but they don't need
to be preserved on function calls.

So when some volatile registers become available, those registers can
always be reallocated regardless of whether SEEN_FUNC is set or not.

Suggested-by: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-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/b04c246874b716911139c04bc004b3b14eed07ef.1641817763.git.christophe.leroy@csgroup.eu