ANDROID: dm-user: Fix the list walk-and-delete code

This needs list_for_each_safe, without which we have a use-after-free
bug when updating the next pointer.

Fixes: 83bf345abc0b ("ANDROID: dm: dm-user: New target that proxies BIOs to userspace")
Suggested-by: Akilesh Kailash <akailash@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: I510ac12e2206a836ae6659bd7d96c0542960b26a
(cherry picked from commit 35f9e6bee13168655141057f8187b8b34c7543d9)