gfs2: Rename dinode_demise to evict_behavior

Rename enum dinode_demise to evict_behavior and its items
SHOULD_DELETE_DINODE to EVICT_SHOULD_DELETE,
SHOULD_NOT_DELETE_DINODE to EVICT_SHOULD_SKIP_DELETE, and
SHOULD_DEFER_EVICTION to EVICT_SHOULD_DEFER_DELETE.

In gfs2_evict_inode(), add a separate variable of type enum
evict_behavior instead of implicitly casting to int.

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