kernel/ucount.c: mark user_header with kmemleak_ignore()

kernel/ucount.c: mark user_header with kmemleak_ignore()

The user_header gets caught by kmemleak with the following splat as
missing a free:

  unreferenced object 0xffff99667a733d80 (size 96):
  comm "swapper/0", pid 1, jiffies 4294892317 (age 62191.468s)
  hex dump (first 32 bytes):
    a0 b6 92 b4 ff ff ff ff 00 00 00 00 01 00 00 00  ................
    01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
     kmemleak_alloc+0x4a/0xa0
     __kmalloc+0x144/0x260
     __register_sysctl_table+0x54/0x5e0
     register_sysctl+0x1b/0x20
     user_namespace_sysctl_init+0x17/0x34
     do_one_initcall+0x52/0x1a0
     kernel_init_freeable+0x173/0x200
     kernel_init+0xe/0x100
     ret_from_fork+0x2c/0x40

The BUG_ON()s are intended to crash so no need to clean up after
ourselves on error there.  This is also a kernel/ subsys_init() we don't
need a respective exit call here as this is never modular, so just white
list it.

Link: http://lkml.kernel.org/r/20170203211404.31458-1-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>