gfs2: Fix debugfs glocks dump

commit 10201655 upstream.

The switch to rhashtables (commit 88ffbf3e) broke the debugfs glock
dump (/sys/kernel/debug/gfs2/<device>/glocks) for dumps bigger than a
single buffer: the right function for restarting an rhashtable iteration
from the beginning of the hash table is rhashtable_walk_enter;
rhashtable_walk_stop + rhashtable_walk_start will just resume from the
current position.

The upstream commit doesn't directly apply to 4.4.y because 4.4.y
doesn't have rhashtable_walk_enter and the following mainline commits:

  92ecd73a  
    gfs2: Deduplicate gfs2_{glocks,glstats}_open
  cc37a627  
    gfs2: Replace rhashtable_walk_init with rhashtable_walk_enter

Other than rhashtable_walk_enter, rhashtable_walk_init can fail.  To
handle the failure case in gfs2_glock_seq_stop, we check if
rhashtable_walk_init has initialized iter->walker; if it has not, we
must not call rhashtable_walk_stop or rhashtable_walk_exit.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>