xprtrdma: Increase maximum number of backchannel requests

Reflects the change introduced in commit 067c4696

 ("NFSv4.1:
Bump the default callback session slot count to 16").

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