LU-16699 osc: Prefer NR_ZONE_WRITE_PENDING

LU-16699 osc: Prefer NR_ZONE_WRITE_PENDING

Linux commit v4.7-5966-g5a1c84b404a7
 mm: remove reclaim and compaction retry approximations

Introduced NR_ZONE_WRITE_PENDING which should be used
in mod_zone_page_state.

Older kernels should fallback to NR_UNSTABLE_NFS
or NR_WRITEBACK.

Test-Parameters: trivial
HPE-bug-id: LUS-11559
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I90f22d4bd56f5986eaa5d4a042a2c8ed31fbf752
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50499
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>