xprtrdma: Fix buffer leak after transport set up failure

This leak has been around forever, and is exceptionally rare.

EINVAL causes mount to fail with "an incorrect mount option was
specified" although it's not likely that one of the mount
options is incorrect. Instead, return ENODEV in this case, as this
appears to be an issue with system or device configuration rather
than a specific mount option.

Some obsolete comments are also removed.

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