gfs2: Avoid dequeuing GL_ASYNC glock holders twice

When a locking request fails, the associated glock holder is
automatically dequeued from the list of active and waiting holders.  For
GL_ASYNC locking requests, this will obviously happen asynchronously
and it can race with attempts to cancel that locking request via
gfs2_glock_dq().  Therefore, don't forget to check if a locking request
has already been dequeued in gfs2_glock_dq().

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