Update extra-iput configure argument description

Update extra-iput configure argument description

Commit 15260c7fdc5ac8fe9fb1797c8e383c665e9e0ccd did not function
as advertised, since the conditional which attempted to make
the configure option --(en|dis)able-linux-d_splice_alias-extra-iput
mandatory on linux checked a variable for the system type which
was not set at the time the check ran.

Subsequent discussion of this behavior produced a consensus that
there is not a need to make the configure option mandatory,
due to the narrow range of kernels affected by the bug in question,
so this follow-up commit just fixes the documentation and removes
the ineffective code.

Change-Id: I36d1f8801d355f33c3132fcab166ea76faab8e87
Reviewed-on: http://gerrit.openafs.org/11710
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>