net: buf: revert disallowing blocking in syswq

Reverts the change made in d4d53010f00cadbb4f89c6d41391937646fc1740
(The changes was moved to another file in a restructuring)

The commit incorrectly assumed that no blocking would be allowed in
the syswq. This has caused issues observed among others
in #77241 and #80167

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>