powerpc/mm: Make clear_fixmap() actually work

The clear_fixmap() routine issues map_page() with flags set to 0.
Currently this causes a BUG_ON() inside the map_page(), as it assumes
that a PTE should be clear before mapping.

This patch makes the map_page() to trigger the BUG_ON() only if the
flags were set.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>