kernel: don't auto attach ubi on read error

kernel: don't auto attach ubi on read error

With a10a204aab26cc ("kernel: make ubi auto-attach check for a tar file
magic") the check for the magic was added without considering a failing
mtd_read(). If the read fails, no check is done and the mount code is
called straight away.

Failing with an error message for such cases seems to me the cleaner way,
as it would allow to spot hidden/workaround issues.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 3716b5e4e6374883c80e61021923733b14cdb011)
(rebased patches)
Signed-off-by: Jo-Philipp Wich <jo@mein.io>