Fix the following sparse warning:

  kernel/user.c:85:19: warning: symbol 'uidhash_table' was not declared.
  Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20200413082146.22737-1-yanaijie@huawei.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>