net: buf: Disallow blocking allocation in syswq

Work items on the syswq should always run-to-completion.

Override the timeout value to always be K_NO_WAIT.

Allocating with K_FOREVER leads to deadlocks if the freeing also happens
from the syswq. Non-zero timeouts are also not nice for the other users
of the syswq.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>