We now always map kernel text with BATs. Neither need to preload
hash with kernel text addresses nor ensure they are never evicted.

This is more or less a revert of commit ee4f2ea4 ("[POWERPC] Fix
32-bit mm operations when not using BATs")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0a0bab7fadd89aa829e33420fbc10d60c59040a7.1606285014.git.christophe.leroy@csgroup.eu