LU-3023 build: fix fuzzy logic in get_root_path()

LU-3023 build: fix fuzzy logic in get_root_path()

Thanks to the call to llapi_is_lustre_mnt(), we are sure that
mnt.mnt_fsname contains ":/".
And, if no other mountpoint is found (len ==0), there is no reason
to abort scanning, so the check and -EINVAL can just be removed.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Change-Id: Id77c41fbf53782d657034f152f724a3bdc83408c
Reviewed-on: http://review.whamcloud.com/5832
Reviewed-by: John Hammond <johnlockwoodhammond@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>