nflog: only increment packets_nobufs when recv() returns an error

nflog: only increment packets_nobufs when recv() returns an error

Errno should only be valid when recv() returns a `-1`, indicating an
error.

I believe the intended behavior here is for packets_nobufs to
be a counter that reports back how many times recv() returns
an ENOBUFS during a packet capture. Because of the existing logic
however, packets_nobufs begins incrementing for every recv() call
once the first ENOBUFS error is seen, since errno is not reset when
there are no errors returned from recv().

Before (counter deviates from strace):

    # tcpdump output
    38069 packets captured
    38069 packets received by filter
    38061 packets dropped by kernel

    # strace output
    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
    26.47    0.282728           7     38067         3 recvfrom

After (counter matches strace):

    # tcpdump output
    38095 packets captured
    38095 packets received by filter
    7 packets dropped by kernel

    # strace output
    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
    27.11    0.258596           6     38096         7 recvfrom