bpfilter Makefile assumes that the system locale is en_US, and the
parsing of objdump output fails.
Set LC_ALL=C and, while at it, rewrite the objdump parsing so it spawns
only 2 processes instead of 7.
Fixes: d2ba09c17a064 ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Matteo Croce <redacted>