lib: check for integer-overflow in nlmsg_reserve()

lib: check for integer-overflow in nlmsg_reserve()

In general, libnl functions are not robust against calling with
invalid arguments. Thus, never call libnl functions with invalid
arguments. In case of nlmsg_reserve() this means never provide
a @len argument that causes overflow.

Still, add an additional safeguard to avoid exploiting such bugs.

Assume that @pad is a trusted, small integer.
Assume that n->nm_size is a valid number of allocated bytes (and thus
much smaller then SIZE_T_MAX).
Assume, that @len may be set to an untrusted value. Then the patch
avoids an integer overflow resulting in reserving too few bytes.

http://git.infradead.org/users/tgr/libnl.git/commit/3e18948f17148e6a3c4255bdeaaf01ef6081ceeb
Fix CVE-2017-0553

Change-Id: Ia9ad5040d866d2cc4c1c76eac5275d66edda338b
Signed-off-by: Cheoleun Moon <chleun.moon@samsung.com>