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>