[Store] Fix CXL capacity metric leak on segment unmount

[stmatengss]

Problem

CommitUnmountSegment only decrements metrics for non-CXL segments:

if (!is_cxl) {
    dec_total_mem_capacity(...);
    dec_total_dram_capacity(...);
}
// CXL branch: nothing decremented ← bug

On CXL mount, both inc_total_mem_capacity and inc_total_cxl_capacity are called. The unmount path never mirrors this, causing the CXL capacity metric to grow monotonically across remount cycles.

Fix

Add an else branch calling dec_total_mem_capacity and dec_total_cxl_capacity for CXL segments, matching the mount path symmetrically.

Impact

• No behavior change for RAM segments
• CXL capacity metrics now correctly track mounted capacity