exfat: fix pointer error checking

Fix missing result check of exfat_build_inode().
And use PTR_ERR_OR_ZERO instead of PTR_ERR.

Signed-off-by: Tetsuhiro Kohada <kohada.t2@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Git-commit: d6c9efd92443b23307995f34246c2374056ebbd8
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: lixiang <lixiang@codeaurora.org>