mempolicy: kill MPOL_F_LOCAL bit
Now the only remaining case of actual 'local' policy faked by
'prefer' policy plus MPOL_F_LOCAL bit is:
A valid 'prefer' policy with a valid 'preferred' node is 'rebind'
to a nodemask which doesn't contains the 'preferred' node, then it
will handle allocation with 'local' policy.
Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the
MPOL_F_LOCAL bit, which could simplify the code much.