parisc: Fix map_pages() to not overwrite existing pte entries

Fix a long-existing small nasty bug in the map_pages() implementation which
leads to overwriting already written pte entries with zero, *if* map_pages() is
called a second time with an end address which isn't aligned on a pmd boundry.
This happens for example if we want to remap only the text segment read/write
in order to run alternative patching on the code. Exiting the loop when we
reach the end address fixes this.

Cc: stable@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>