gfs2: Clean up glock demote logic

The logic for determining when to demote a glock in glock_work_func(),
introduced in commit 7cf8dcd3

 ("GFS2: Automatically adjust glock min
hold time"), doesn't make sense: inode glocks have a minimum hold time
that delays demotion, while all other glocks are expected to be demoted
immediately.  Instead of demoting non-inode glocks immediately,
glock_work_func() schedules glock work for them to be demoted, however.
Get rid of that unnecessary indirection.

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