LU-9859 libcfs: deprecate libcfs_debug_vmsg2

LU-9859 libcfs: deprecate libcfs_debug_vmsg2

Since 2.6.36, Linux' vsprintf has supported %pV
which supports "recursive sprintf" - exactly the task
that libcfs_debug_vmsg2 aims to provide.

Instead of calling libcfs_debug_vmsg2(), we can put the fmt and
args in a 'struct va_format', and pass the address of that structure
to the "%pV" format.

So do this to remove all users of libcfs_debug_vmsg2().

Linux-commit: 0fe922e1eca8e2850f0e6c535a14ba7414ca73c2

Change-Id: I6952ca8fdb619423639734aab1a30f4635b089cc
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35224
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>