diag: Copy length and buffer locally to send to userspace client

To prevent possible use after free case of accessing buffers
when mhi goes down copy the buffer content and length locally
which can be used to copy to userspace.

Change-Id: Iada4ab1756a2b3792a45547eec4e47869c8eb94d
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>