LoongArch: BPF: Fix unconditional bswap instructions

We can see that "bswap32: Takes an unsigned 32-bit number in either big-
or little-endian format and returns the equivalent number with the same
bit width but opposite endianness" in BPF Instruction Set Specification,
so it should clear the upper 32 bits in "case 32:" for both BPF_ALU and
BPF_ALU64.

[root@linux fedora]# echo 1 > /proc/sys/net/core/bpf_jit_enable
[root@linux fedora]# modprobe test_bpf

Before:
test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 ret 1460850314 != -271733879 (0x5712ce8a != 0xefcdab89)FAIL (1 times)
test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 ret -1460850316 != 271733878 (0xa8ed3174 != 0x10325476)FAIL (1 times)

After:
test_bpf: #313 BSWAP 32: 0x0123456789abcdef -> 0xefcdab89 jited:1 4 PASS
test_bpf: #317 BSWAP 32: 0xfedcba9876543210 -> 0x10325476 jited:1 4 PASS

Fixes: 4ebf9216 ("LoongArch: BPF: Support unconditional bswap instructions")
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>