FROMLIST: android: binder: Refactor prev and next buffer into a helper function

(from https://patchwork.kernel.org/patch/9928605/

)

Use helper functions buffer_next and buffer_prev instead
of list_entry to get the next and previous buffers.

Bug: 36007193
Change-Id: I422dce84afde3d2138a6d976593b109a9cc49003
Signed-off-by: Sherry Yang <sherryy@android.com>