xprtrdma: Inline XDR chunk encoder functions

Micro-optimization: Save the cost of three function calls during
transport header encoding.

These were "noinline" before to generate more meaningful call stacks
during debugging, but this code is now pretty stable.

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