MIPS: BPF: Quit clobbering callee saved registers in JIT code.

MIPS: BPF: Quit clobbering callee saved registers in JIT code.

[ Upstream commit 1ef0910cfd681f0bd0b81f8809935b2006e9cfb9 ]

If bpf_needs_clear_a() returns true, only actually clear it if it is
ever used.  If it is not used, we don't save and restore it, so the
clearing has the nasty side effect of clobbering caller state.

Also, don't emit stack pointer adjustment instructions if the
adjustment amount is zero.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Steven J. Hill <steven.hill@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15745/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>