compat: Detect and use inet_frags->frags_work.

compat: Detect and use inet_frags->frags_work.

Kernels 3.17 and newer have a work queue to evict old fragments, while
older kernel versions use an LRU in the fast path; see upstream commit
b13d3cbfb8e8 ("inet: frag: move eviction of queues to work queue").
This commit fixes the version checking so that rather than enabling the
code for either of these approaches using version checks, it is
triggered based on the presence of the work queue in "struct inet_frags".

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>