LUDOC-37 kernel: use /etc/modprobe.d/lustre.conf

LUDOC-37 kernel: use /etc/modprobe.d/lustre.conf

Update the documentaion to store module parameters in the
/etc/modprobe.d/lustre.conf file to make the newer distro
modutils happy.

Early versions of modprobe did not care what filenames were
used in /etc/modprobe.d/, which lead to confusion if old
versions of files were located there (e.g. Lustre.orig or
Lustre~).  Newer versions of modprobe (at least since FC12,
but possibly earlier) require module parameter files to have
a ".conf" suffix in order to be parsed.

The /etc/modprobe.d/ directory has been supported since at
least module-init-utils 3.2 (RHEL5) and may have been supported
before that time.  This means that all supported distros can
use /etc/modprobe.d/lustre.conf for Lustre-specific parameters.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic885f610b3d9b16eb211d4a2d971930f53647bec