powerpc/bpf ppc32: Fix BPF_SUB when imm == 0x80000000

powerpc/bpf ppc32: Fix BPF_SUB when imm == 0x80000000

[ Upstream commit 548b762763b885b81850db676258df47c55dd5f9 ]

Special case handling of the smallest 32-bit negative number for BPF_SUB.

Fixes: 51c66ad849a703 ("powerpc/bpf: Implement extended BPF on PPC32")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7135360a0cdf70adedbccf9863128b8daef18764.1633464148.git.naveen.n.rao@linux.vnet.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>