btrfs: delete BUG_ON in btrfs_init_locked_inode()

The purpose of the BUG_ON is not clear. The helper btrfs_grab_root()
could return a NULL in case args->root would be a NULL or if there are
zero references. Then we check if the root pointer stored in the inode
still exists.

The whole call chain is for iget:

btrfs_iget
  btrfs_iget_path
    btrfs_iget_locked
      iget5_locked
	btrfs_init_locked_inode

which is called from many contexts where we the root pointer is used and
we can safely assume has enough references.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5571,7 +5571,6 @@ static int btrfs_init_locked_inode(struct inode *inode, void *p)
 	BTRFS_I(inode)->location.type = BTRFS_INODE_ITEM_KEY;
 	BTRFS_I(inode)->location.offset = 0;
 	BTRFS_I(inode)->root = btrfs_grab_root(args->root);
-	BUG_ON(args->root && !BTRFS_I(inode)->root);
 
 	if (args->root && args->root == args->root->fs_info->tree_root &&
 	    args->ino != BTRFS_BTREE_INODE_OBJECTID)