IB/hfi1: Fix user SDMA racy user request claim

The user SDMA in-use claim bit is in the structure that gets zeroed out
once the claim is made.  Move the request in-use flag into its own bit
array and use that for atomic claims.  This cleans up the claim code and
removes any race possibility.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>