fix pcap_write function

fix pcap_write function

when use pcap cli to capture pcakets into two files rx01.pcap && rx02.pcap,
the first time:
1)pcap rx trace on max 100 intfc any file rx01.pcap
2)......the process of capture data to buffer......
3)pcap rx trace off

the second time:
4)pcap rx trace on max 100 intfc any file rx02.pcap
5)......the process of capture data to buffer......
6)pcap rx trace off

the pcap_write function bug in this two lines
pm->n_packets_captured = 0;
if (pm->n_packets_captured >= pm->n_packets_to_capture) referring to calling pcap_close()
will result in that the twice pcap cli both writes the packets
into rx01.pcap, but nothing into rx02.pcap. Beside, the rx02.pcap
file will not be created.

solution: separate the pcap_close() out of pcap_write()

Change-Id: Iedeb46f9cf0a4cb12449fd75a4014f95f3bb3fa8
Signed-off-by: Jack Xu <jack.c.xu@ericsson.com>