libbpf: Fix register in PT_REGS MIPS macros

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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/bpf_tracing.h

···
 #define PT_REGS_PARM5(x) ((x)->regs[8])
 #define PT_REGS_RET(x) ((x)->regs[31])
 #define PT_REGS_FP(x) ((x)->regs[30]) /* Works only with CONFIG_FRAME_POINTER */
-#define PT_REGS_RC(x) ((x)->regs[1])
+#define PT_REGS_RC(x) ((x)->regs[2])
 #define PT_REGS_SP(x) ((x)->regs[29])
 #define PT_REGS_IP(x) ((x)->cp0_epc)
 
···
 #define PT_REGS_PARM5_CORE(x) BPF_CORE_READ((x), regs[8])
 #define PT_REGS_RET_CORE(x) BPF_CORE_READ((x), regs[31])
 #define PT_REGS_FP_CORE(x) BPF_CORE_READ((x), regs[30])
-#define PT_REGS_RC_CORE(x) BPF_CORE_READ((x), regs[1])
+#define PT_REGS_RC_CORE(x) BPF_CORE_READ((x), regs[2])
 #define PT_REGS_SP_CORE(x) BPF_CORE_READ((x), regs[29])
 #define PT_REGS_IP_CORE(x) BPF_CORE_READ((x), cp0_epc)