librbd/cache/pwl/ssd: fix use-after-free on C_BlockIORequest

librbd/cache/pwl/ssd: fix use-after-free on C_BlockIORequest

In setup_schedule_append() function, its first expression
will cause the req to be deleted, and subsequent use of
the variable req becomes an illegal operation. And due to
delete, rep->m_image_ctx will be empty, so it lead to
segfault in AbstractWriteLog::get_context().
So pass the `req` into `schedule_append()` function.

Fixes: https://tracker.ceph.com/issues/50951
Signed-off-by: Hualong Feng <hualong.feng@intel.com>