riscv, bpf: Fix out-of-bounds issue when preparing trampoline image

In the Linux kernel, the following vulnerability has been resolved:

riscv, bpf: Fix out-of-bounds issue when preparing trampoline image

We get the size of the trampoline image during the dry run phase and

allocate memory based on that size. The allocated image will then be

populated with instructions during the real patch phase. But after

commit 26ef208c209a ("bpf: Use arch_bpf_trampoline_size"), the `im`

argument is inconsistent in the dry run and real patch phase. This may

cause emit_imm in RV64 to generate a different number of instructions

when generating the 'im' address, potentially causing out-of-bounds

issues. Let's emit the maximum number of instructions for the "im"

address during dry run to fix this problem.