The net_buf_unref() API will unref all associated fragments, so it's
wrong to assume that no fragment destroy callbacks would be called
when the head buf is unreffed. Instead the right expectation is that
all fragments get unrefed as well.
Change-Id: I00530e4c1cfa5dddbfcbeab9609347873010ff4d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>