unifdef: use memcpy instead of strncpy

unifdef: use memcpy instead of strncpy

commit 38c7b224ce22c25fed04007839edf974bd13439d upstream.

New versions of gcc reasonably warn about the odd pattern of

	strncpy(p, q, strlen(q));

which really doesn't make sense: the strncpy() ends up being just a slow
and odd way to write memcpy() in this case.

There was a comment about _why_ the code used strncpy - to avoid the
terminating NUL byte, but memcpy does the same and avoids the warning.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[dongkyun.s: cherry-pick stable linux-4.9.y commit 37370fadd2da for gcc 9 build]

Change-Id: I2f005aa7c6642d76d818f1fda7722f345095b8a9