drivers/media/platform/sti/delta/delta-ipc.c: fix read buffer overflow

drivers/media/platform/sti/delta/delta-ipc.c: fix read buffer overflow

The single caller passes a string to delta_ipc_open, which copies with a
fixed size larger than the string.  So it copies some random data after
the original string the ro segment.

If the string was at the end of a page it may fault.

Just copy the string with a normal strcpy after clearing the field.

Found by a LTO build (which errors out)
because the compiler inlines the functions and can resolve
the string sizes and triggers the compile time checks in memcpy.

In function `memcpy',
    inlined from `delta_ipc_open.constprop' at linux/drivers/media/platform/sti/delta/delta-ipc.c:178:0,
    inlined from `delta_mjpeg_ipc_open' at linux/drivers/media/platform/sti/delta/delta-mjpeg-dec.c:227:0,
    inlined from `delta_mjpeg_decode' at linux/drivers/media/platform/sti/delta/delta-mjpeg-dec.c:403:0:
/home/andi/lsrc/linux/include/linux/string.h:337:0: error: call to `__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
    __read_overflow2();

Link: http://lkml.kernel.org/r/20171222001212.1850-1-andi@firstfloor.org
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Hugues FRUCHET <hugues.fruchet@st.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>