btrfs: tree-checker: add inode extref checks

Like inode refs, inode extrefs have a variable length name, which means
we have to do a proper check to make sure no header nor name can exceed
the item limits.

The check itself is very similar to check_inode_ref(), just a different
structure (btrfs_inode_extref vs btrfs_inode_ref).

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>