radix-tree: remove restriction on multi-order entries

radix-tree: remove restriction on multi-order entries

Now that sibling pointers are handled explicitly, there is no purpose
served by restricting the order to be >= RADIX_TREE_MAP_SHIFT.

Signed-off-by: Matthew Wilcox <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Kirill Shutemov <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Neil Brown <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>