Revert "ashmem: switch to ->read_iter"

This reverts commit 8a2af064 ("ashmem: switch to ->read_iter")

This change resulted in ashmem file descriptors now getting labeled as
system_server_tmpfs instead of ashmem_device.

SELinux is rejecting access to file descriptors labeled as
system_server_tmpfs and this is causing the battery settings app to crash.

It does not look feasible to get SELinux to accept system_server_tmpfs
labeled file descriptors as it is used too broadly.
So for now revert this change until we can find a clean way to fix this.

Change-Id: I5710c37f1204a296736b0e22bdaf12fe292d9c62
Signed-off-by: Liam Mark <lmark@codeaurora.org>