LU-16671 osc: fix unstable pages for short IO

LU-16671 osc: fix unstable pages for short IO

Unstable pages was written with theoretical support for
short IO (ie, no bulk, data-in-rpc, LU-1757), but since the
short IO code wasn't merged until years later, they were
probably never tested together.  And when you do, it
crashes.

In truth, short IO has no separate pages to be tracked,
which is why this is crashing.  This means that small write
RPCs won't be tracked in unstable pages, but that's a very
minor limitation and unlikely to cause trouble.  (and since
RPC allocations are not 'pages', they're just malloc'ed,
there's no good way to track them anyway)

Fixes: 70f092a ("LU-1757 brw: add short io osc/ost transfer.")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I34b09f8324424c3ff0b0c09c86f01c938b643e37
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50451
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>