compat: Detect and use inet_frags->lock.

compat: Detect and use inet_frags->lock.

Prior to ab1c724f6330 ("inet: frag: use seqlock for hash rebuild")
upstream, a rwlock was used when rebuilding inet_frags. Rather than
using a version check to detect this, search for it in the header and
enable the code based on whether it exists.

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