kernel/vfs: Fix locking behavior in acquire_vnode and add a missing ASSERT.

vnode deletion is prevent either by owning a reference to the node,
or by being at least a reader of the vnodes lock. Thus, in acquire_vnode,
we have to inc_ref_count while still holding the lock in order to prevent
a race.

This function is used so rarely and quite deep inside FS drivers that
I'm  not sure this race would ever have been a problem. Nonetheless
the old code was incorrect.

src/system/kernel/fs/vfs.cpp

@@ -899,6 +899,8 @@ remove_vnode_from_mount_list(struct vnode* vnode, struct fs_mount* mount)
 static struct vnode*
 lookup_vnode(dev_t mountID, ino_t vnodeID)
 {
+	ASSERT_READ_LOCKED_RW_LOCK(&sVnodeLock);
+
 	struct vnode_hash_key key;
 
 	key.device = mountID;
@@ -3894,12 +3896,9 @@ get_vnode(fs_volume* volume, ino_t vnodeID, void** _privateNode)
 extern "C" status_t
 acquire_vnode(fs_volume* volume, ino_t vnodeID)
 {
-	struct vnode* vnode;
-
-	rw_lock_read_lock(&sVnodeLock);
-	vnode = lookup_vnode(volume->id, vnodeID);
-	rw_lock_read_unlock(&sVnodeLock);
+	ReadLocker nodeLocker(sVnodeLock);
 
+	struct vnode* vnode = lookup_vnode(volume->id, vnodeID);
 	if (vnode == NULL)
 		return B_BAD_VALUE;