ANDROID: GKI: net: add vendor hooks for 'struct nf_conn' lifecycle

Some vendors want to add a field when a 'sruct nf_conn' is added so give a
hook to handle this.  Any memory allocated when
trace_android_rvh_nf_conn_alloc() is called needs to be freed when
trace_android_rvh_nf_conn_free() is called.

Note, if trace_android_rvh_nf_conn_alloc() fails, be sure to be able to
handle this in trace_android_rvh_nf_conn_free(), but that should not be
an issue as that needs to be addressed in vendor code that runs for
'struct nf_conn' objects that have been created before the vendor code
is loaded no matter what.

Bug: 171013716
Signed-off-by: Vignesh Saravanaperumal <vignesh1.s@samsung.com>
Change-Id: I4d2b025196a3df7ba4adec313c90483811cac728