mds/Migrator.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
os/LFNIndex.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/OSD.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/OSDMap.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/PG.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/ReplicatedPG.cc: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/SnapMapper.cc: use empty() instead of size() == 0

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