staging: erofs: formatting fix to NULL comparison

This patch does not change the logic, it only
corrects the formatting and checkpatch checks by
to NULL comparison.

The patch fixes 5 checks of type:
"Comparison to NULL could be written".

Signed-off-by: Pavel Zemlyanoy <zemlyanoy@ispras.ru>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>