hypertable/CephBroker.cc: use int64_t instead of uint64_t

hypertable/CephBroker.cc: use int64_t instead of uint64_t

The usage of uint64_t to get the return value of ceph_lseek() will
lead to loss of the error code returned from the function. This will
break error case detection. Use a signed variable and cast before
offset returned instead.

Fix for:

[src/client/hypertable/CephBroker.cc:209]: (style) Checking if
 unsigned variable 'offset=ceph_lseek(cmount,fdata->fd,0,SEEK_CUR)' is less
 than zero.
[src/client/hypertable/CephBroker.cc:244]: (style) Checking if
 unsigned variable 'offset=(uint64_t)ceph_lseek(cmount,fdata->fd,0,SEEK_CUR)'
 is less than zero.

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