Fix an issue caused by variable re-declaration in inner scope

Since btf is declared in inner scope, btf is null in outer scope and BTF
was always disabled.

This CL also makes NetBpfLoad buildable with -Wshadow flag

Test: adb log | grep NetBpfLoad
Change-Id: I511f21a3620ccfdadd797786e087d0d142c816ed
1 file changed