LU-14427 libcfs: restore LNET_DUMP_ON_PANIC functionality.

LU-14427 libcfs: restore LNET_DUMP_ON_PANIC functionality.

The functionality enabled by --enable-panic-dumplog was inadvertently
removed in Commit ae0704381efc ("LU-9859 libcfs: merge linux-debug.c
into debug.c")

Restore it.

While we are there, add conditional-compliation for other code that is
only needed when this is enabled.

Test-Parameters: trivial
Fixes: ae0704381efc ("LU-9859 libcfs: merge linux-debug.c into debug.c")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: If85882045c66e54ff8493396589d4ecbf13f8f3d
Reviewed-on: https://review.whamcloud.com/41488
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>