dmapool: add alloc/free performance test

dmapool: add alloc/free performance test

Patch series "dmapool enhancements", v4.

Time spent in dma_pool alloc/free increases linearly with the number of
pages backing the pool.  We can reduce this to constant time with minor
changes to how free pages are tracked.

This patch (of 12):

Provide a module that allocates and frees many blocks of various sizes and
report how long it takes.  This is intended to provide a consistent way to
measure how changes to the dma_pool_alloc/free routines affect timing.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Keith Busch <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Tony Battersby <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>