maple_tree: add test to replicate low memory race conditions

Add new callback fields to the userspace implementation of struct
kmem_cache.  This allows for executing callback functions in order to
further test low memory scenarios where node allocation is retried.

This callback can help test race conditions by calling a function when a
low memory event is tested.

Link: https://lkml.kernel.org/r/20240812190543.71967-2-sidhartha.kumar@oracle.com


Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>