bpf, x86: Simplify the parsing logic of structure parameters

[ Upstream commit 7f788049

 ]

Extra_nregs of structure parameters and nr_args can be
added directly at the beginning, and using a flip flag
to identifiy structure parameters. Meantime, renaming
some variables to make them more sense.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230105035026.3091988-1-pulehui@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Stable-dep-of: 2b5dcb31

 ("bpf, x64: Fix tailcall infinite loop")
Signed-off-by: Sasha Levin <sashal@kernel.org>