libbpf: Fix register in PT_REGS MIPS macros

libbpf: Fix register in PT_REGS MIPS macros

[ Upstream commit 1acf8f90ea7ee59006d0474275922145ac291331 ]

The o32, n32 and n64 calling conventions require the return
value to be stored in $v0 which maps to $2 register, i.e.,
the register 2.

Fixes: c1932cd ("bpf: Add MIPS support to samples/bpf.")
Signed-off-by: Jerry Crunchtime <jerry.c.t@web.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/43707d31-0210-e8f0-9226-1af140907641@web.de
Signed-off-by: Sasha Levin <sashal@kernel.org>