gfs2: Return enum evict_behavior from gfs2_upgrade_iopen_glock

In case an iopen glock cannot be upgraded, function
gfs2_upgrade_iopen_glock() needs to communicate to gfs2_evict_inode()
whether deleting the inode should be deferred or skipped altogether.
Change the function to return the appropriate enum evict_behavior value
to indicate that.

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