The atexit() and signal() callbacks both need to only use signalsafe
functions - that excludes the use of assert. So simplify
fork_helper_exit_handler() and children_exit_handler().

__lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
0x00007fd630883d2b in _L_lock_13840 () from /lib/x86_64-linux-gnu/libc.so.6
0x00007fd630881df8 in __GI___libc_realloc (oldmem=0xfcb010, bytes=88) at malloc.c:3025
0x00007fd63087111b in _IO_vasprintf (result_ptr=0x7fff35dc4780, format=<optimised out>, args=args@entry=0x7fff35dc4658) at vasprintf.c:84
0x00007fd630852907 in ___asprintf (string_ptr=string_ptr@entry=0x7fff35dc4780, format=format@entry=0x7fd63097f718 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n") at asprintf.c:35
0x00007fd63082dd92 in __assert_fail_base (fmt=0x7fd63097f718 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x40cff5 "ret == 0", file=file@entry=0x4108d1 "igt_core.c", line=line@entry=872, function=function@entry=0x410ea0 <__PRETTY_FUNCTION__.8052> "children_exit_handler") at assert.c:57
0x00007fd63082dee2 in __GI___assert_fail (assertion=assertion@entry=0x40cff5 "ret == 0", file=file@entry=0x4108d1 "igt_core.c", line=line@entry=872, function=function@entry=0x410ea0 <__PRETTY_FUNCTION__.8052> "children_exit_handler") at assert.c:101
0x000000000040b03f in children_exit_handler (sig=<optimised out>) at igt_core.c:872
0x000000000040b089 in call_exit_handlers (sig=2) at igt_core.c:1029 fatal_sig_handler (sig=2) at igt_core.c:1053 <signal handler called>
0x00007fd6308bfe63 in __libc_fork () at ../nptl/sysdeps/unix/sysv/linux/x86_64/../fork.c:130
0x00007fd630bd6045 in __fork () at ../nptl/sysdeps/unix/sysv/linux/pt-fork.c:25
0x000000000040c51a in __igt_fork () at igt_core.c:900
0x00000000004036c2 in forking_evictions (ops=0x614360 <fault_ops>, surface_size=1048576, flags=5, trash_surfaces=<optimised out>, working_surfaces=338, fd=4) at eviction_common.c:203
test_forking_evictions (size=1048576, flags=5, count=338, fd=4) at gem_userptr_blits.c:1086
main (argc=1, argv=0x7fff35dc5328) at gem_userptr_blits.c:1478

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>