osd_types: pg_t::is_split(): use unsigned for bits operations

osd_types: pg_t::is_split(): use unsigned for bits operations

this silences the warning of

The result of the '<<' expression is undefined

quote from n3337, §5.8 expr.shift:

If E1 has an unsigned type, the value ... (is deterministic)...
Otherwise, if E1 has a signed type and non-negative value, and E1 x 2^E2
is representable in the result type, then that is the resulting value;
otherwise, the behavior is undefined.

Signed-off-by: Kefu Chai <kchai@redhat.com>