Add tracking of access times

Add the atime field, which tracks the last access time. Only reads
update the atime.

**BREAKING CHANGE**: One less extent available per block, 4 more bytes
for node names.

src/node.rs

@@ -14,10 +14,12 @@ pub struct Node {
     pub ctime_nsec: u32,
     pub mtime: u64,
     pub mtime_nsec: u32,
-    pub name: [u8; 222],
+    pub atime: u64,
+    pub atime_nsec: u32,
+    pub name: [u8; 226],
     pub parent: u64,
     pub next: u64,
-    pub extents: [Extent; (BLOCK_SIZE as usize - 272)/16],
+    pub extents: [Extent; (BLOCK_SIZE as usize - 288)/16],
 }
 
 impl Node {
@@ -40,15 +42,17 @@ impl Node {
             ctime_nsec: 0,
             mtime: 0,
             mtime_nsec: 0,
-            name: [0; 222],
+            atime: 0,
+            atime_nsec: 0,
+            name: [0; 226],
             parent: 0,
             next: 0,
-            extents: [Extent::default(); (BLOCK_SIZE as usize - 272)/16],
+            extents: [Extent::default(); (BLOCK_SIZE as usize - 288)/16],
         }
     }
 
     pub fn new(mode: u16, name: &str, parent: u64, ctime: u64, ctime_nsec: u32) -> syscall::Result<Node> {
-        let mut bytes = [0; 222];
+        let mut bytes = [0; 226];
         if name.len() > bytes.len() {
             return Err(syscall::Error::new(syscall::ENAMETOOLONG));
         }
@@ -64,10 +68,12 @@ impl Node {
             ctime_nsec: ctime_nsec,
             mtime: ctime,
             mtime_nsec: ctime_nsec,
+            atime: ctime,
+            atime_nsec: ctime_nsec,
             name: bytes,
             parent: parent,
             next: 0,
-            extents: [Extent::default(); (BLOCK_SIZE as usize - 272)/16],
+            extents: [Extent::default(); (BLOCK_SIZE as usize - 288)/16],
         })
     }
 
@@ -85,7 +91,7 @@ impl Node {
     }
 
     pub fn set_name(&mut self, name: &str) -> syscall::Result<()> {
-        let mut bytes = [0; 222];
+        let mut bytes = [0; 226];
         if name.len() > bytes.len() {
             return Err(syscall::Error::new(syscall::ENAMETOOLONG));
         }