blockd: don't reparse blob msg in the vlist callbacks

blockd: don't reparse blob msg in the vlist callbacks

ubus message is parsed in the block_hotplug() which fills all the struct
device fields. Once that is done there is no need to parse original
message again - it's enough to get required data from the struct.

This also fixes handling messages with "autofs" set to 0. They were
incorrectly interpreted due to the missing blobmsg_get_u32().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>