gfs2: Initialize iopen glock holder for new inodes

In gfs2_init_inode_once, initialize inode->i_iopen_gh.gh_gl to NULL:
otherwise, when gfs2_inode_lookup fails, the iopen glock holder can
remain unset and iget_failed can end up accessing random memory.

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