spfs: add an option for mounting overlayfs layers using syscalls

Teach spfs to mount /spfs using direct overlayfs syscalls.
This is provided as an opt-in feature so that users can test this
alongside the current "mount"-based implementation. If no issues are
encountered then this can be made the default behavior in the future.

This approach avoids the command-line length limits that are present
when using the "mount" command to create the /spfs mount.

Related-to: #968
Related-to: #1164

Author: davvid

crates/spfs/src/config.rs

@@ -367,6 +367,11 @@ pub struct Filesystem {
     /// directly in the annotation layer.
     #[serde(default = "Filesystem::default_annotation_size_limit")]
     pub annotation_size_limit: usize,
+
+    /// Use direct syscalls when performing overlayfs mounts when true.
+    /// Use the "mount" command when false. Defaults to false.
+    #[serde(default)]
+    pub use_mount_syscalls: bool,
 }
 
 impl Filesystem {