[GFS2] Remove a cast, tidy gfs2_inode_attr_in

The remains of the changes for Jan Engelhardt's third email. Remove
a cast and tidy up gfs2_inode_attr_in.

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