IB/hfi1: Improve local kmem_cache_alloc performance

IB/hfi1: Improve local kmem_cache_alloc performance

Performance analysis shows that the cache callback function
sdma_kmem_cache_ctor contributes to 1/2 of the kmem_cache_allocs
time.

Since all of the fields in the allocated data structure are initialized
in the code path, remove the _ctor function.

Reviewed-by: Mike Marciniszyn <[email protected]>
Signed-off-by: Michael J. Ruhl <[email protected]>
Signed-off-by: Dennis Dalessandro <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>