client: fix quota signed/unsigned warning

client/Client.cc: In member function 'bool Client::is_quota_bytes_exceeded(Inode*, uint64_t)':
client/Client.cc:10393:66: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (quota->max_bytes && (rstat->rbytes + new_bytes) > quota->max_bytes)

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