gfs2: further simplify gfs2_evict_inode with new func evict_should_delete

This patch further simplifies function gfs2_evict_inode() by adding a
new function evict_should_delete. The function may also lock the inode
glock.

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