blobmsg: fix wrong payload len passed from blobmsg_check_array

blobmsg: fix wrong payload len passed from blobmsg_check_array

Fix incorrect use of blob_raw_len() on passed blobmsg to
blobmsg_check_array_len()  introduced in commit b0e21553ae8c ("blobmsg:
add _len variants for all attribute checking methods") by using correct
blobmsg_len().

This wrong (higher) length was then for example causing issues in
procd's instance_config_parse_command() where blobmsg_check_attr_list()
was failing sanity checking of service command, thus resulting in the
startup failures of some services like collectd, nlbwmon and samba4.

Ref: http://lists.infradead.org/pipermail/openwrt-devel/2019-December/020840.html
Fixes: b0e21553ae8c ("blobmsg: add _len variants for all attribute checking methods")
Reported-by: Hannu Nyman <hannu.nyman@welho.com>
Tested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Petr Štetiar <ynezz@true.cz>