Add BPF_F_SLEEPABLE flag support to LSM

[epompeii]

This adds support for the BPF_F_SLEEPABLE flag for LSM : https://lore.kernel.org/netdev/20200827220114.69225-3-alexei.starovoitov@gmail.com/

It follows the same tack as XDP multi-buffer support: #519

Internally, this option follows the libbpf convention and is represented as lsm.s: https://elixir.bootlin.com/linux/latest/source/tools/lib/bpf/libbpf.c#L8560

In the public API, this is exposed via the eBPF lsm macro as an optional sleepable = "bool" argument.

#[lsm(name = "lsm_bprm_check_security", sleepable = "true")]
pub fn bprm_check_security(ctx: LsmContext) -> i32 {
    match try_bprm_check_security(ctx) {
        Ok(ret) => ret,
        Err(ret) => ret,
    }
}

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[alessandrod]

Thanks for doing this! See comments

[alessandrod]

Thank you! Lgtm. Can you please rebase? I've fixed the CI failure on main.