staging: lustre: remove task_t typedef

Use struct task_struct instead.

Also remove the unused libcfs_current() function, while I noticed it
doing this conversion.

Cc: Peng Tao <tao.peng@emc.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>