init: Check chroot return value in sysupgrade_exec_upgraded()

init: Check chroot return value in sysupgrade_exec_upgraded()

chroot() can fail and its return value should be checked against, in that case
do an exit() since this is a fatal condition that we cannot recover from.

Fixes: 63789e51ed91 ("init: add support for sysupgrades triggered from preinit")
Reviewed-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>