devlink: remove a dubious assumption in fmsg dumping

Build bot detects that err may be returned uninitialized in
devlink_fmsg_prepare_skb(). This is not really true because
all fmsgs users should create at least one outer nest, and
therefore fmsg can't be completely empty.

That said the assumption is not trivial to confirm, so let's
follow the bots advice, anyway.

This code does not seem to have changed since its inception in
commit 1db64e87 ("devlink: Add devlink formatted message (fmsg) API")

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230124035231.787381-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>