gfs2: Fix "Introduce flag for glock holder auto-demotion"

Function demote_incompat_holders iterates over the list of glock holders
with list_for_each_entry, and it then sometimes removes the current
holder from the list.  This will get the loop stuck; we must use
list_for_each_entry_safe instead.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>