mds/MDS.cc: fix improper use of negative value

Change set_client_incarnation to accept only AO_t since atomic_t.set()
only accept AO_t which is unsigned.

Make sure the result of mdsmap->get_inc_gid() is positive to avoid
passing negative values to set_client_incarnation().

Fix for:

CID 1238883 (#1 of 1): Improper use of negative value (NEGATIVE_RETURNS)
 negative_returns: this->incarnation is passed to a parameter that
 cannot be negative. [show details]

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