don't panic if out of disk space when extending a directory.

kernel/fs.c

@@ -573,6 +573,7 @@ dirlookup(struct inode *dp, char *name, uint *poff)
 }
 
 // Write a new directory entry (name, inum) into the directory dp.
+// Returns 0 on success, -1 on failure (e.g. out of disk blocks).
 int
 dirlink(struct inode *dp, char *name, uint inum)
 {
@@ -597,7 +598,7 @@ dirlink(struct inode *dp, char *name, uint inum)
   strncpy(de.name, name, DIRSIZ);
   de.inum = inum;
   if(writei(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
-    panic("dirlink");
+    return -1;
 
   return 0;
 }

kernel/sysfile.c

@@ -279,8 +279,13 @@ create(char *path, short type, short major, short minor)
       panic("create dots");
   }
 
-  if(dirlink(dp, name, ip->inum) < 0)
-    panic("create: dirlink");
+  if(dirlink(dp, name, ip->inum) < 0){
+    // oops. we don't need ip after all.
+    ip->nlink = 0;
+    iupdate(ip);
+    iunlockput(ip);
+    ip = 0;
+  }
 
   iunlockput(dp);