if_bridge: count the packets bridge_pfil() drops on error

[netchild]

Summary

bridge_pfil() dropped a packet on error paths without incrementing
the error counter:

• a failed m_pullup() or M_PREPEND(),
• a malformed header,
• a fragmentation failure

Route those exits through the existing bad: label and bump the filtered
interface's input or output error counter.

Also add error counter handling in bridge_fragment().

Assisted-by: Claude Code (Fable 5)

[netchild]

The following part may need to get a rework for this change... investigating...

		default:
			if (V_pfil_onlyip)
				goto bad;