ashmem: switch to ->read_iter

And use the proper VFS helper for using the backing file.

Also make sure we hold ashmem_lock while updating f_pos to make sure that
it's not racy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Git-commit: 8a2af064
Git-repo: https://github.com/torvalds/linux


Change-Id: I1121fd9756b05f2a7ad25ee417404aa56a4f2a9a
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>