xprtrdma: Eliminate INLINE_THRESHOLD macros

Clean up: r_xprt is already available everywhere these macros are
invoked, so just dereference that directly.

RPCRDMA_INLINE_PAD_VALUE is no longer used, so it can simply be
removed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>