ext4: fix inode use after free in ext4_end_io_rsv_work()

ext4: fix inode use after free in ext4_end_io_rsv_work()

In ext4_io_end_defer_completion(), check if io_end->list_vec is empty to
avoid adding an io_end that requires no conversion to the
i_rsv_conversion_list, which in turn prevents starting an unnecessary
worker. An ext4_emergency_state() check is also added to avoid attempting
to abort the journal in an emergency state.

Additionally, ext4_put_io_end_defer() is refactored to call
ext4_io_end_defer_completion() directly instead of being open-coded.
This also prevents starting an unnecessary worker when EXT4_IO_END_FAILED
is set but data_err=abort is not enabled.

This ensures that the check in ext4_put_io_end_defer() is consistent with
the check in ext4_end_bio(). Otherwise, we might add an io_end to the
i_rsv_conversion_list and then call ext4_finish_bio(), after which the
inode could be freed before ext4_end_io_rsv_work() is called, triggering
a use-after-free issue.

Fixes: ce51afb8cc5e ("ext4: abort journal on data writeback failure if in data_err=abort mode")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250708111504.3208660-1-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>