hppfs: fix the return value of get_inode()

In case of error, the function get_inode() returns ERR_PTR().
But the users hppfs_lookup() and hppfs_fill_super() use NULL
test for check the return value, not IS_ERR(), so we'd better
change the return value of get_inode() to NULL instead of
ERR_PTR().

dpatch engine is used to generated this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>