tmpfs - Use atomic_fetchadd_long() to allocate inode numbers

tmpfs - Use atomic_fetchadd_long() to allocate inode numbers

* Since DFly inode numbers are 64 bits wide, tmpfs just increments a
  per-mount counter to allocate a new inode.

* We had some old code in tmpfs to allocate inode numbers which wrapped
  a tmpfs mount lock around the increment.  This is totally unnecessary.
  Use atomic_fetchadd_long() instead.

Reported-by: mjg