[release/8.0] Don't assume all S_IFREG files are seekable (#122188)

Backport of #120736 to release/8.0

/cc @rzikm

## Customer Impact

None, the change is in Debug build only and doesn't affect shipped
product.

## Regression

No, test failures started after adding/updating AzureLinux3 OS in CI
matrix.

## Testing

CI passes.

## Risk

Low, can be considered test-only change as the changes don't propagate
to shipped product.

---------

Co-authored-by: Radek Zikmund <r.zikmund.rz@gmail.com>

Author: github-actions[bot]

src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Unix.cs

@@ -344,7 +344,10 @@ private bool Init(string path, FileMode mode, FileAccess access, FileShare share
                     // and for regular files (most common case)
                     // avoid one extra sys call for determining whether file can be seeked
                     _canSeek = NullableBool.True;
-                    Debug.Assert(Interop.Sys.LSeek(this, 0, Interop.Sys.SeekWhence.SEEK_CUR) >= 0);
+
+                    // we exclude 0-length files from the assert because those may may be pseudofiles
+                    // (e.g. /proc/net/route) and these are not seekable on all systems
+                    Debug.Assert(status.Size == 0 || Interop.Sys.LSeek(this, 0, Interop.Sys.SeekWhence.SEEK_CUR) >= 0);
                 }
 
                 fileLength = status.Size;