[FS#1523] block-mount: unnecessary remounts after extroot switch

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Andreas (ndvour) 

Attached to Project - OpenWrt/LEDE Project
Summary - block-mount: unnecessary remounts after extroot switch
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Very Low
Priority - Very Low
Reported Version - lede-17.01
Due in Version - Undecided
Due Date - Undecided
Details - Environment: arm_cortex-a9_vfpv3, Linksys WRT1200AC, LEDE Reboot 17.01.4 r3560-79f57e422d

Description:
A USB flash drive has been configured with an ext4 partition to be used as external overlay, following the steps at https://openwrt.org/docs/guide-user/additional-software/extroot_configuration precisely. After the extroot switch, block tries to remount partitions that have already been mounted, generating the following errors in the logs:
Sat Apr 28 12:25:00 2018 daemon.err block: /dev/ubiblock0_0 is already mounted on /rom
Sat Apr 28 12:25:00 2018 daemon.err block: /dev/sdb1 is already mounted on /overlay


More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=1523