staging: lustre: replace static value with define

This patch replace the value '80' used in several files in the lustre source code
with a define LUSTRE_MDT_MAXNAMELEN.

This value is used in 4 different structures as the maximum len for a service name.
According to the comments, these names follow a convention which make it possible
to use the same define for LCS, LSS, LCF, and LSF.

Signed-off-by: Tristan Lelong <tristan@lelong.xyz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>