xprtrdma: Make rpcrdma_{un}map_one() into inline functions

These functions are called in a loop for each page transferred via
RDMA READ or WRITE. Extract loop invariants and inline them to
reduce CPU overhead.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Devesh Sharma <Devesh.Sharma@Emulex.Com>
Tested-by: Meghana Cheripady <Meghana.Cheripady@Emulex.Com>
Tested-by: Veeresh U. Kokatnur <veereshuk@chelsio.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>