pipe: iovec: Fix OOB read in pipe_read()

Previous upstream *stable* fix 14f81062 was incomplete.

A local process can trigger a system crash with an OOB read on buf.
This occurs when the state of buf gets out of sync. After an error in
pipe_iov_copy_to_user() read_pipe may exit having updated buf->offset
but not buf->len. Upon retrying pipe_read() while in
pipe_iov_copy_to_user() *remaining will be larger than the space left
after buf->offset e.g. *remaing = PAGE_SIZE, buf->len = PAGE_SIZE,
buf->offset = 0x300.

This is fixed by not updating the state of buf->offset until after the
full copy is completed, similar to how pipe_write() is implemented.

For stable kernels < 3.16.

Bug: 27721803
Change-Id: Iefffbcc6cfd159dba69c31bcd98c6d5c1f21ff2e
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>