powerpc/603: Inconditionally use task PGDIR in ITLB misses

Now that modules exec page tables are preallocated, the instruction
TLBmiss handler can use task PGDIR inconditionally.

Also revise the identification of user vs kernel user space by doing
a calculation instead of a comparison: Get the segment number and
subtract the number of the first kernel segment. The result is
positive for kernel addresses and negative for user addresses,
which means that upper 2 bits are 0 for kernel and 3 for user.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/9a3242162ad2faab8019c698e501b326a126ee9e.1724173828.git.christophe.leroy@csgroup.eu