vmm: memory_manager: Add SEEK_DATA-based extent iterator

Adds next_data_extent: a streaming helper that returns the next
populated extent within a window of a file descriptor using
lseek(SEEK_DATA) / lseek(SEEK_HOLE). Used by subsequent commits to walk
the snapshot file extent-by-extent without collecting the full extent
list.

Returns an error on fds or filesystems without SEEK_HOLE support so the
caller can fall back to a dense write path.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/Cargo.toml

@@ -103,5 +103,8 @@ vmm-sys-util = { workspace = true, features = ["with-serde"] }
 zbus = { version = "5.14.0", optional = true }
 zerocopy = { workspace = true, features = ["alloc", "derive"] }
 
+[dev-dependencies]
+tempfile = "3"
+
 [lints]
 workspace = true