LU-13476 llite: Fix lock ordering in pagevec_dirty

LU-13476 llite: Fix lock ordering in pagevec_dirty

In vvp_set_pagevec_dirty lock order between i_pages and
lock_page_memcg was inverted with the expectation that
no other users would conflict.

However in vvp_page_completion_write the call to
test_clear_page_writeback does expect to be able
to lock_page_memcg then lock i_pages which appears
to conflict with the original analysis.

The reported case shows as RCU stalls with
vvp_set_pagevec_dirty blocked attempting to lock i_pages.

Fixes: a7299cb012f ("LU-9920 vvp: dirty pages with pagevec")
HPE-bug-id: LUS-8798
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I47c2107ddbef4a76325928e982abfc0ea666f39b
Reviewed-on: https://review.whamcloud.com/38317
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>