powerpc: Rename USER_ESID_BITS* to ESID_BITS*

Now we use ESID_BITS of kernel address to build proto vsid. So rename
USER_ESIT_BITS to ESID_BITS

Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: <stable@vger.kernel.org> [v3.8]