block: re-discover mtd devices on extroot mount retry

block: re-discover mtd devices on extroot mount retry

When retrying extroot mounting after waiting for the underlying device to
appear, the internal device cache was rebuilt without mtd devices, leading
to the following error:

    block: extroot: unable to lookup root device /dev/...

The extroot mount procedure needs access to the actual mtd root device to
temporarily mount it for uuid file verification.

Fix this error by rebuilding the cache with mtd devices to allow the
`check_extroot()` procedure to find the internal root device.

Fixes: FS#2701
Fixes: fb0700f ("block: support hierarchical mount/umount")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>