lsns: fix the memory leak.

lsns: fix the memory leak.

==28129== 96 bytes in 3 blocks are possibly lost in loss record 1 of 4
==28129==    at 0x4837B65: calloc (vg_replace_malloc.c:752)
==28129==    by 0x405C83: xcalloc (xalloc.h:60)
==28129==    by 0x405C83: netnsid_cache_add (lsns.c:389)
==28129==    by 0x405C83: get_netnsid (lsns.c:486)
==28129==    by 0x405C83: read_process (lsns.c:549)
==28129==    by 0x403FB4: read_processes (lsns.c:586)
==28129==    by 0x403FB4: main (lsns.c:1417)
==28129==
==28129== 119,664 (384 direct, 119,280 indirect) bytes in 1 blocks are definitely lost in loss record 4 of 4
==28129==    at 0x4837B65: calloc (vg_replace_malloc.c:752)
==28129==    by 0x4055F5: xcalloc (xalloc.h:60)
==28129==    by 0x4055F5: read_process (lsns.c:516)
==28129==    by 0x403FB4: read_processes (lsns.c:586)
==28129==    by 0x403FB4: main (lsns.c:1417)