quota: Fix error codes in v2_read_file_info()

v2_read_file_info() returned -1 instead of proper error codes on error.
Luckily this is not easily visible from userspace as we have called
->check_quota_file shortly before and thus already verified the quota
file is sane. Still set the error codes to proper values.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>