b=20563 Fix fid_flatten() after 1 trillion SEQ numbers

b=20563 Fix fid_flatten() after 1 trillion SEQ numbers

Fix the fid_flatten() function to properly handle FID mapping to
64-bit inode numbers, after the first 1 trillion SEQ numbers have
been granted out.  Even with CMD this would only happen after
1024 MDTs have each had 1B client mounts, so there is little risk
of introducing collisions as a result of this change, and at worst
this is a client-local phenomenon that is not persistent.