brd: fix sleeping function called from invalid context in brd_insert_page()

brd: fix sleeping function called from invalid context in brd_insert_page()

__xa_cmpxchg() is called with rcu_read_lock(), and it will allocate
memory if necessary.

Fix the problem by moving rcu_read_lock() after __xa_cmpxchg(), meanwhile,
it still should be held before xa_unlock(), prevent returned page to be
freed by concurrent discard.

Fixes: bbcacab2e8ee ("brd: avoid extra xarray lookups on first write")
Reported-by: syzbot+ea4c8fd177a47338881a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/685ec4c9.a00a0220.129264.000c.GAE@google.com/
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250630112828.421219-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>