[GFS2] Extended attribute code style changes

As per Jan Engelhardt's request and also a few of my own. It has
been possible to add a few most const to the code as a result of
the change in gfs2_ea_name2type.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>