IB/hfi1: Implement SDMA-side buffer caching

Add support for caching of user buffers used for SDMA
transfers. This change improves performance by
avoiding repeatedly pinning the pages of buffers, which
are being re-used by the application.

While the cost of the pinning operation has been made
heavier by adding the extra code to search the cache tree,
re-allocate pages arrays, and future cache evictions,
that cost will be amortized against the savings when the
same buffer is re-used. It is also worth noting that in
most cases, the cost of pinning should be much lower due
to the buffer already being in the cache.

Reviewed-by: Dennis Dalessandro <[email protected]>
Reviewed-by: Dean Luick <[email protected]>
Signed-off-by: Mitko Haralanov <[email protected]>
Signed-off-by: Jubin John <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>

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