error-inject: use IS_ERR() check for debugfs_create_file()

error-inject: use IS_ERR() check for debugfs_create_file()

debugfs_create_file() returns an error pointer on failure, never NULL, so
the !file check in ei_debugfs_init() never triggers and the
debugfs_remove() cleanup cannot run.

Use IS_ERR() and propagate the actual error via PTR_ERR().

Link: https://lore.kernel.org/20260514193214.2432769-1-ingyujang25@korea.ac.kr
Signed-off-by: Ingyu Jang <ingyujang25@korea.ac.kr>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>