LU-14541 llite: add rw_seq_cst_vs_drop_caches

LU-14541 llite: add rw_seq_cst_vs_drop_caches

Add a reproducer (rw_seq_cst_vs_drop_caches) for the read/write vs
drop_caches sequnetial consistency violation described in
LU-14541. Add an always excepted test (sanityn test_16f) to run
rw_seq_cst_vs_drop_caches.

Lustre-change: https://review.whamcloud.com/47243/
Lustre-commit: 8b9185d500b1e3ccdf66c84c1cbba82a0e392e69

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I557ae7386b38214110a4d85ba0515e95fed7a11e
Reviewed-on: https://review.whamcloud.com/47272
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>