Get wrong loop device's name when the loop device's name was more than 9.

[ev4ngel]

https://github.com/nanhantianyi/rpi-backup/blob/4a1124554d89e03bee360ef6b399a297b81f4e73/back.sh#L60C48-L60C48

The backup script didn't work for me at first.I tried to print every line's result and find out the loop device's name was loop15 ,so i added a '+' after loop[0-9] phrase,and it did work.

device=`kpartx -va $loopdevice | sed -E 's/.*(loop[0-9]+)p.*/\1/g' | head -1`
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