client: fix warnings

client/Client.cc: In member function 'int Client::_read(Fh*, int64_t, uint64_t, ceph::bufferlist*)':
warning: client/Client.cc:5893:27: comparison between signed and unsigned integer expressions [-Wsign-compare]
client/Client.cc: In member function 'int Client::_write(Fh*, int64_t, uint64_t, const char*)':
warning: client/Client.cc:6235:30: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Sage Weil <sage@inktank.com>