buf_buddy_relocate(): The ut_time_us() function is needed for
statistics, calculating the total time spent on relocating blocks.
Until now, we invoked ut_time_us() every time buf_buddy_relocate() was
called. Fix: Only call ut_time_us() when the block can be relocated.
After this fix, the reported relocated_usec will no longer include the
time for the page_hash lookup and for acquiring the block mutex.

Approved by Sunny Bains on IM