main/postmarketos-initramfs: fix resize bug
Fix root partition not getting resized if subpartitions are used (how we
install postmarketOS to all Androids). Run the resize logic if the
partition starts with /dev/dm too, e.g. on the OnePlus 6:
partition=/dev/dm-1
The bug has already been fixed in postmarketOS edge with MR 4204, patch
"main/postmarketos-initramfs: optimise find_*_partition". However this
patch contains more refactoring so here is a separate patch with just
the fix.
I've verified that this fixes it with OP6.
Fix in master is part of 89fba12f.
Fixes #2235 (closed) for v23.06.