Some tools in btrfs-progs utilize ioctl 'BTRFS_IOC_TREE_SEARCH' and
ioctl 'BTRFS_IOC_TREE_SEARCH_V2' to look up metadata btree for what
they want, and several tools in fact only look for one certain type,
where they set a certain value for both 'sk->min_type' and 'sk->max_type'.

For example,
if we want to get the information of block groups, the current btrfs
searches extent_tree and returns not only block groups's items, but also
EXTENT_ITEM's items which could cost a large amount of user's buffer,
and tools then needs to read the buffer and spends several loops to
pick up what they want.

This lets the above two ioctl only return the certain type of items
that tools wants.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>