gfs2: Get rid of demote_ok checks

The demote_ok glock operation is only still used to prevent the inode
glocks of the "jindex" and "rindex" directories from getting recycled
while they are still referenced by sdp->sd_jindex and sdp->sd_rindex.
However, the LRU walking code will no longer recycle glocks which are
referenced, so the demote_ok glock operation is obsolete and can be
removed.

Each of a glock's holders in the gl_holders list is holding a reference
on the glock, so when the list of holders isn't empty in demote_ok(),
the existing reference count check will already prevent the glock from
getting released.  This means that demote_ok() is obsolete as well.

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