block: make bioset_exit() fully resilient against being called twice

block: make bioset_exit() fully resilient against being called twice

[ Upstream commit aea9d92cfd7becc5c1b3398b882c956c26b59f51 ]

Most of bioset_exit() is fine being called twice, as it clears the
various allocations etc when they are freed. The exception is
bio_alloc_cache_destroy(), which does not clear ->cache when it has
freed it.

This isn't necessarily a bug, but can be if buggy users does call the
exit path more then once, or with just a memset() bioset which has
never been initialized. dm appears to be one such user.

Fixes: 94b92e98906a ("bio: add allocation cache abstraction")
Link: https://lore.kernel.org/linux-block/YpK7m+14A+pZKs5k@casper.infradead.org/
Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>