[GFS2] selinux support

This adds support to GFS2 for selinux extended attributes. There is a
known bug in gfs2_ea_get() which is believed to be independant of this
patch. Further patches will follow once that bug is fixed in order to
make GFS2 use as much of the generic eattr infrastructure as possible.

Signed-off-by: Ryan O'Hara <rohara@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>