libceph: use list_for_each_entry_safe

Use list_for_each_entry_safe() instead of list_for_each_safe() to
simplify the code.

Signed-off-by: Geliang Tang <geliangtang@163.com>
[idryomov@gmail.com: nuke call to list_splice_init() as well]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>