osdc: warning: comparison between signed and unsigned integer expressions

osdc: warning: comparison between signed and unsigned integer expressions

The following warning appears during make. Fixed the review comments too.
[ 28%] Building CXX object src/mds/CMakeFiles/mds.dir/__/osdc/Journaler.cc.o
./src/osdc/Journaler.cc: In member function ‘void Journaler::_do_flush(unsigned int)’:
./src/osdc/Journaler.cc:643:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (newlen < len) {
         ~~~~~~~^~~~~

Signed-off-by: Jos Collin <jcollin@redhat.com>