lib: json_print: rework 'new_json_obj' drop FILE* argument

As Stephen Hemminger mentioned on the last submission the new_json_obj
function is always called with fp == stdout, so right now, there's no
need of this extra argument.

The background for the rework is the following:
The ip monitor didn't call `new_json_obj` (even for in non json context),
so the static FILE* _fp variable wasn't initialized, thus raising a
SIGSEGV in ipaddress.c. This patch should fix this issue for good, new
paths won't have to call `new_json_obj`.

How to reproduce:

$ ip -t mon label link
(gdb) bt
.#0  _IO_vfprintf_internal (s=s@entry=0x0, format=format@entry=0x45460d “%d: “, ap=ap@entry=0x7fffffff7f18) at vfprintf.c:1278
.#1  0x0000000000451310 in color_fprintf (fp=0x0, attr=<optimized out>, fmt=0x45460d “%d: “) at color.c:108
.#2  0x000000000044a856 in print_color_int (t=t@entry=PRINT_ANY, color=color@entry=4294967295, key=key@entry=0x4545fc “ifindex”,
    fmt=fmt@entry=0x45460d “%d: “, value=<optimized out>) at ip_print.c:132
.#3  0x000000000040ccd2 in print_int (value=<optimized out>, fmt=0x45460d “%d: “, key=0x4545fc “ifindex”, t=PRINT_ANY) at ip_common.h:189
.#4  print_linkinfo (who=<optimized out>, n=0x7fffffffa380, arg=0x7ffff77a82a0 <_IO_2_1_stdout_>) at ipaddress.c:1107
.#5  0x0000000000422e13 in accept_msg (who=0x7fffffff8320, ctrl=0x7fffffff8310, n=0x7fffffffa380, arg=0x7ffff77a82a0 <_IO_2_1_stdout_>) at ipmonitor.c:89
.#6  0x000000000044c58f in rtnl_listen (rtnl=0x672160 <rth>, handler=handler@entry=0x422c70 <accept_msg>, jarg=0x7ffff77a82a0 <_IO_2_1_stdout_>)
    at libnetlink.c:761
.#7  0x00000000004233db in do_ipmonitor (argc=<optimized out>, argv=0x7fffffffe5a0) at ipmonitor.c:310
.#8  0x0000000000408f74 in do_cmd (argv0=0x7fffffffe7f5 “mon”, argc=3, argv=0x7fffffffe588) at ip.c:116
.#9  0x0000000000408a94 in main (argc=4, argv=0x7fffffffe580) at ip.c:311

Fixes: 6377572f ("ip: ip_print: add new API to print JSON or regular format output")
Reported-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>
diff --git a/include/json_print.h b/include/json_print.h
index 44cf5ac..b6ce1f9 100644
--- a/include/json_print.h
+++ b/include/json_print.h
@@ -29,13 +29,11 @@
 	PRINT_ANY = 4,
 };
 
-void new_json_obj(int json, FILE *fp);
+void new_json_obj(int json);
 void delete_json_obj(void);
 
 bool is_json_context(void);
 
-void set_current_fp(FILE *fp);
-
 void fflush_fp(void);
 
 void open_json_object(const char *str);