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 <dennis.dalessandro@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>