libxtables: Register only the highest revision extension

libxtables: Register only the highest revision extension

When fully registering extensions, ignore all consecutive ones with same
name and family value. Since commit b3ac87038f4e4 ("libxtables: Make
sure extensions register in revision order"), one may safely assume the
list of pending extensions has highest revision numbers first. Since
iptables is only interested in the highest revision the kernel supports,
registration and compatibility checks may be skipped once the first
matching extension in pending list has validated.

Signed-off-by: Phil Sutter <phil@nwl.cc>