gfs2: prevent NULL pointer dereference during unmount

gfs2: prevent NULL pointer dereference during unmount

When flushing out outstanding glock work during an unmount, gfs2_log_flush()
can be called when sdp->sd_jdesc has already been deallocated and sdp->sd_jdesc
is NULL.  Commit 35264909e9d1 ("gfs2: Fix NULL pointer dereference in
gfs2_log_flush") added a check for that to gfs2_log_flush() itself, but it
missed the sdp->sd_jdesc dereference in gfs2_log_release().  Fix that.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202604071139.HNJiCaAi-lkp@intel.com/
Fixes: 35264909e9d1 ("gfs2: Fix NULL pointer dereference in gfs2_log_flush")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>