locking/rwlocks: introduce write_lock_nested

locking/rwlocks: introduce write_lock_nested

In preparation for converting bit_spin_lock to rwlock in zsmalloc so that
multiple writers of zspages can run at the same time but those zspages are
supposed to be different zspage instance.  Thus, it's not deadlock.  This
patch adds write_lock_nested to support the case for LOCKDEP.

Link: https://lkml.kernel.org/r/20211115185909.3949505-8-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>