[fix] Alert Settings: Disk usage Is healthy: False

[stargieg]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Alert Settings: Disk usage
Is healthy: False

"resources":
{
    "memory":{"total":124080128,"shared":491520,"free":59867136,"cached":25333760,"available":48177152,"buffered":0},
    "cpus":1,
    "disk":[
        {"filesystem":"\/dev\/root","available_bytes":0,"mount_point":"\/rom","used_percent":100,"size_bytes":4980736,"used_bytes":4980736},
        {"filesystem":"\/dev\/mtdblock6","available_bytes":356352,"mount_point":"\/overlay","used_percent":40,"size_bytes":589824,"used_bytes":233472}
    ],
    "load":[0,0.02,0],"swap":{"free":0,"total":0}
},

ubus call system board

{
	"kernel": "6.6.73",
	"hostname": "bmu-ap-ac1580",
	"system": "Qualcomm Atheros QCA9558 ver 1 rev 0",
	"model": "OpenMesh A60",
	"board_name": "openmesh,a60",
	"rootfs_type": "squashfs",
	"release": {
		"distribution": "OpenWrt",
		"version": "24.10.0",
		"revision": "r28427-6df0e3d02a",
		"target": "ath79/generic",
		"description": "OpenWrt 24.10.0 r28427-6df0e3d02a",
		"builddate": "1738624177"
	}
}

Description of Changes

disk_usage exclude mountpoint /rom