Consolidate duplicated device parsing logic in grow-ssd script

[Copilot]

Addresses feedback from PR #44 regarding duplicated device parsing patterns in the grow-ssd service script.

Changes

• Unified parsing logic: Device path parsing (disk/partition extraction) now happens once upfront instead of in separate branches
• Simplified control flow: When PKNAME exists, only the disk variable is overridden rather than re-parsing the entire device path
• Reduced duplication: Eliminated ~15 lines of redundant pattern matching for NVMe and non-NVMe partition schemes

Before/After

Before: Two parallel case statements with duplicate regex patterns

if [ -n "$PKNAME" ]; then
    DISK="/dev/$PKNAME"
    case "$SSD_DEV" in
        /dev/nvme*p[0-9]*) PART=$(sed -E 's/.*p([0-9]+)$/\1/');;
        /dev/*[0-9]) PART=$(sed -E 's/.*[^0-9]([0-9]+)$/\1/');;
    esac
else
    case "$SSD_DEV" in
        /dev/nvme*p[0-9]*) 
            DISK=$(sed -E 's/p[0-9]+$//')
            PART=$(sed -E 's/.*p([0-9]+)$/\1/');;
        /dev/*[0-9])
            DISK=$(sed -E 's/[0-9]+$//')
            PART=$(sed -E 's/.*[^0-9]([0-9]+)$/\1/');;
    esac
fi

After: Single parsing pass with optional override

# Parse disk and partition from device path
case "$SSD_DEV" in
    /dev/nvme*p[0-9]*) 
        DISK=$(sed -E 's/p[0-9]+$//')
        PART=$(sed -E 's/.*p([0-9]+)$/\1/');;
    /dev/*[0-9])
        DISK=$(sed -E 's/[0-9]+$//')
        PART=$(sed -E 's/.*[^0-9]([0-9]+)$/\1/');;
esac

# Override disk if parent device exists
PKNAME=$(lsblk -no pkname "$SSD_DEV" 2>/dev/null || echo "")
if [ -n "$PKNAME" ]; then
    DISK="/dev/$PKNAME"
fi

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[kristapsk]

Comment is not correct, this will work not only with LVM or whatever, in most cases lsblk output will always succeed (e.g. lsblk -no pkname /dev/sda1 2>/dev/null || echo "" -> sda).