netfilter: nf_conntrack_extend: introduce __nf_ct_ext_exist()

some users of nf_ct_ext_exist() know ct->ext isn't NULL. For these users, the
check for ct->ext isn't necessary, the function __nf_ct_ext_exist() can be
used instead.

the type of the return value of nf_ct_ext_exist() is changed to bool.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>