iomap: prevent ioend merge when io_private differs

iomap: prevent ioend merge when io_private differs

Different io_private values indicate distinct completion contexts that
must not be merged together, as this could leak or corrupt the private
data associated with each ioend.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260713074206.1768006-1-yi.zhang@huaweicloud.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>