sget() is pessimistic and assumes it won't find a superblock - and as a
result it always pre-allocates a new one before even searching for an
existing entry.  If it finds an existing one, it free's the extra one it
just allocated.

Especially with /proc usage it usually _does_ find an existing
superblock, so this is all extra work.  It should allocate a new one
only if it couldn't find an old one (and re-search).

The same way as iget() does it.