fix(exec): move ext2 I/O outside interrupt-disabled section

[ryanbreen]

Summary

Fixes intermittent TCP test failures in CI caused by exec syscall disabling interrupts during ext2 filesystem I/O.

Root Cause

The entire sys_execv_with_frame() was wrapped in without_interrupts(), causing:

• ext2 filesystem I/O (path resolution, inode reads, file content) to run with interrupts disabled
• Timer interrupts blocked during VirtIO disk operations
• Scheduler starvation
• TCP packet processing delays

Changes

1. Restructure exec syscall - Only wrap the critical frame manipulation in without_interrupts(), not the ELF loading
2. Increase TCP retry budget - MAX_LOOPBACK_RETRIES from 3 to 10 for CI resilience

Technical Details

Before:

pub fn sys_execv_with_frame(...) -> SyscallResult {
    x86_64::instructions::interrupts::without_interrupts(|| {
        // ALL code here - including ext2 I/O
    })
}

After:

pub fn sys_execv_with_frame(...) -> SyscallResult {
    // Preparation and ELF loading WITH interrupts enabled
    let elf_data = load_elf_from_ext2(...);  // ext2 I/O works properly
    
    // Only critical section disables interrupts
    x86_64::instructions::interrupts::without_interrupts(|| {
        // Frame manipulation only
    })
}

Test plan

• All 217 boot stages pass locally
• TCP tests (stages 85-127) pass
• ext2 exec tests (stages 179-184) pass

🤖 Generated with Claude Code