LoongArch: BPF: Save return address register ra to t0 before trampoline

LoongArch: BPF: Save return address register ra to t0 before trampoline

Modify the build_prologue() function to ensure the return address
register ra is saved to t0 before entering trampoline operations.
This change ensures the accurate return address handling when a BPF
program calls another BPF program, preventing errors in the BPF-to-BPF
call chain.

Cc: stable@vger.kernel.org
Fixes: 677e6123e3d2 ("LoongArch: BPF: Disable trampoline for kernel module function trace")
Signed-off-by: Chenghao Duan <duanchenghao@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>