ext4: fix the error handling process in extents_kunit_init).

ext4: fix the error handling process in extents_kunit_init).

The error processing in extents_kunit_init() is improper, causing
resource leakage.
Reconstruct the error handling process to prevent potential resource
leaks

Fixes: cb1e0c1d1fad ("ext4: kunit tests for extent splitting and conversion")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20260330133035.287842-4-yebin@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>