powerpc: Fail ioremap() instead of silently ignoring flags when PAGE_USER is set

Calling ioremap() with _PAGE_USER (or _PAGE_PRIVILEDGE unset)
is wrong. Loudly fail the call to ioremap() instead of blindly
clearing the flags.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b6dd5485ad00d2aafd2bb9b7c2c4eac3ebf2cdaf.1695659959.git.christophe.leroy@csgroup.eu