daemon: Do not error out when deduplication fails due to ENOSPC.

This solves a problem whereby if /gnu/store/.links had enough entries,
ext4's directory index would be full, leading to link(2) returning
ENOSPC.

* nix/libstore/optimise-store.cc (LocalStore::optimisePath_): Upon
ENOSPC from link(2), print a message and return instead of throwing a
'SysError'.

nix/libstore/optimise-store.cc

@@ -148,10 +148,23 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path, InodeHa
             inodeHash.insert(st.st_ino);
             return;
         }
-        if (errno != EEXIST)
-            throw SysError(format("cannot link `%1%' to `%2%'") % linkPath % path);
+
+	switch (errno) {
+	case EEXIST:
 	    /* Fall through if another process created ‘linkPath’ before
 	       we did. */
+	    break;
+
+	case ENOSPC:
+	    /* On ext4, that probably means the directory index is full.  When
+	       that happens, it's fine to ignore it: we just effectively
+	       disable deduplication of this file.  */
+	    printMsg(lvlInfo, format("cannot link `%1%' to `%2%': %m") % linkPath % path);
+	    return;
+
+	default:
+            throw SysError(format("cannot link `%1%' to `%2%'") % linkPath % path);
+	}
     }
 
     /* Yes!  We've seen a file with the same contents.  Replace the