ntfs: return errors from ntfs_attr_readall

ntfs_attr_readall() currently loses the failure reason for attribute
lookup, allocation, and read failures by returning NULL. Return ERR_PTR()
with the original error instead.

The reparse parser can then propagate allocation and I/O errors without
treating them as filesystem corruption.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>