LU-13783 osc: handle removal of NR_UNSTABLE_NFS

LU-13783 osc: handle removal of NR_UNSTABLE_NFS

In Linux 5.8 the NR_UNSTABLE_NFS page counters are go.  All pages that
have been writen but are not yet safe are now counted in NR_WRITEBACK.

So change osc_page to count in NR_WRITEBACK, but if NR_UNSTABLE_NFS
still exists in the kernel, use a #define to direct the updates to
that counter.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I49cbc267fafaee949f45b2e559511aedcf4d8fed
Reviewed-on: https://review.whamcloud.com/39260
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>