lib/test_maple_tree: add testing for maple tree

This is a test suite that uses the radix test infrastructure.  It has been
split into its own commit to allow for easier review of the maple tree
code.

The testing includes:
- Allocation of nodes
- gfp flag allocation checks
- Expansion & contraction of tree
- preallocation checks
- tree navigation by next/prev
- tree navigation by iterators (mas_for_each, etc)
- Number of nodes for a given number of entries
- Generic tree construction tests
- Addition and removal of entries in forward and reverse numerical indexes
- gap searching both forward and reverse
- Combining gaps by overwriting entries in different ways
- splitting right-most node
- splitting left-most node
- overwriting multiple slots
- overwriting across different levels of the tree
- overwriting the middle of a tree
- causing a 3-way split up to the root by overwriting the last slot and
  first slot of different nodes and spanning different levels
- RCU stress testing of the tree with threads
- Duplication of the tree by entry count
- Tests which were generated by fuzzers have been added.
- A large number of tests which come from recording crashing in a VM and
  reconstructing the tree (see check_erase2_set())

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Liam R. Howlett <[email protected]>
Tested-by: Yu Zhao <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: howlett