xfrm: use memcpy to suppress gcc phony buffer overflow warning.

This bug is reported from below link:
https://bugzilla.redhat.com/show_bug.cgi?id=982761

An simplified command from its original reproducing method in bugzilla:
ip xfrm state add src 10.0.0.2 dst 10.0.0.1 proto ah spi 0x12345678 auth md5 12
will cause below spew from gcc.

Reported-by: Sohny Thomas <sthomas@linux.vnet.ibm.com>
1 file changed