staging/rdma/hfi1: convert buffers allocated atomic to per cpu

Profiling has shown the the atomic is a performance issue
for the pio hot path.

If multiple cpus allocated an sc's buffer, the cacheline
containing the atomic will bounce from L0 to L0.

Convert the atomic to a percpu variable.

Reviewed-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>