gfs2: simplify the logic in gfs2_evict_inode

Now that we've factored out the deleted and undeleted dinode cases
in gfs2_evict_inode, we can greatly simplify the logic. Now the
function is easy to read and understand.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>