staging: erofs: use explicit unsigned int type

Fix coding style issue "Prefer 'unsigned int' to bare use of 'unsigned'"
detected by checkpatch.pl.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>