kernel/sys.c: replace do_brk with do_brk_flags in comment of prctl_set_mm_map()

Replace do_brk with do_brk_flags in comment of prctl_set_mm_map(), since
do_brk was removed in following commit.

Fixes: bb177a73

 ("mm: do not bug_on on incorrect length in __mm_populate()")
Signed-off-by: Liao Pingfang <liao.pingfang@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/1600650751-43127-1-git-send-email-wang.yi59@zte.com.cn


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>