Maple Tree: add new data structure

Maple Tree: add new data structure

The maple tree is an RCU-safe range based B-tree designed to use modern
processor cache efficiently.  There are a number of places in the kernel that a
non-overlapping range-based tree would be beneficial, especially one with a
simple interface.  If you use an rbtree with other data structures to improve
performance or an interval tree to track non-overlapping ranges, then this is
for you.

The tree has a branching factor of 10 for non-leaf nodes and 16 for leaf nodes.
With the increased branching factor, it is significantly shorter than the
rbtree so it has fewer cache misses.  The removal of the linked list between
subsequent entries also reduces the cache misses and the need to pull in the
previous and next VMA during many tree alterations.

The first user that is covered in this patch set is the vm_area_struct, where
three data structures are replaced by the maple tree: the augmented rbtree, the
vma cache, and the linked list of VMAs in the mm_struct.  The long term goal is
to reduce or remove the mmap_lock contention.

The plan is to get to the point where we use the maple tree in RCU mode.
Readers will not block for writers.  A single write operation will be allowed
at a time.  A reader re-walks if stale data is encountered. VMAs would be RCU
enabled and this mode would be entered once multiple tasks are using the
mm_struct.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: David Howells <dhowells@redhat.com>