staging: lustre: libcfs: Define pr_fmt() to include relevant name

Define a pr_fmt() for tracefile.c
Remove redundant prefix 'Lustre' from pr_*() calls.

Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>