CVE-2024-42079 gfs2: Fix NULL pointer dereference in gfs2_log_flush

In the Linux kernel, the following vulnerability has been resolved:

gfs2: Fix NULL pointer dereference in gfs2_log_flush

In gfs2_jindex_free(), set sdp->sd_jdesc to NULL under the log flush
lock to provide exclusion against gfs2_log_flush().

In gfs2_log_flush(), check if sdp->sd_jdesc is non-NULL before
dereferencing it. Otherwise, we could run into a NULL pointer
dereference when outstanding glock work races with an unmount
(glock_work_func -> run_queue -> do_xmote -> inode_go_sync ->
gfs2_log_flush).