afs: Add tracing for cell refcount and active user count

Add a tracepoint to log the cell refcount and active user count and pass in
a reason code through various functions that manipulate these counters.

Additionally, a helper function, afs_see_cell(), is provided to log
interesting places that deal with a cell without actually doing any
accounting directly.

Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/proc.c

@@ -129,7 +129,7 @@ static int afs_proc_cells_write(struct file *file, char *buf, size_t size)
 		}
 
 		if (test_and_set_bit(AFS_CELL_FL_NO_GC, &cell->flags))
-			afs_unuse_cell(net, cell);
+			afs_unuse_cell(net, cell, afs_cell_trace_unuse_no_pin);
 	} else {
 		goto inval;
 	}