ipxfrm: use alloca to allocate stack space

Summary:
Android note: Fixes the following with clang

external/iproute2/ip/ipxfrm.c:563:8: error: fields must have a constant size: 'variable length array in structure' extension
will never be supported
                char key[algo->alg_key_len / 8];
                     ^
external/iproute2/ip/ipxfrm.c:582:8: error: fields must have a constant size: 'variable length array in structure' extension
will never be supported
                char key[algo->alg_key_len / 8];
                     ^

Clang doesn't support the gcc extension for embeddeding flexible arrays
inside of structures.  Use the slightly more portable alloca().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D135