staging: erofs: Modify conditional checks

Fix coding style alignment issues detected by checkpatch.pl
Use ! for NULL test rather than explicitly comparing to NULL.

Signed-off-by: Bhagyashri P. Dighole <digholebhagyashri@gmail.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>