powerpc: Only make kernel text pages of linear mapping executable

Commit bc033b63 ("powerpc/mm: Fix
attribute confusion with htab_bolt_mapping()") moved the check for
whether we should make pages of the linear mapping executable from
htab_bolt_mapping into its callers, including htab_initialize.
A side-effect of this is that the decision is now made once for
each contiguous section in the LMB array rather than for each page
individually.  This can often mean that the whole of the linear
mapping ends up being executable.

This reverts to the previous behaviour, where individual pages are
checked for being part of the kernel text or not, by moving the check
back down into htab_bolt_mapping.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>