CInode.h: use !old_inodes.empty() instead of size()

CInode.h: use !old_inodes.empty() instead of size()

Use empty() since it should be prefered as it has, following the
standard, a constant time complexity regardless of the containter
type. The same is not guaranteed for size().

warning from cppcheck was:
[src/mds/journal.cc:470]: (performance) Possible inefficient
  checking for 'old_inodes' emptiness.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>