dsp-kernel: Avoid accessing user memory outside uaccess routines

If shell fd is passed as 0 or -1,fastrpc_get_args should not access
file pointer from userspace outside uaccess routines, instead use
copy_from_user to create a local copy of the file, to ensure
proper access.

Change-Id: I9a0fd70c045f445a94b4f147005df47d5bcefc18
Signed-off-by: Anandu Krishnan E <quic_anane@quicinc.com>