Allow zero size updates. This makes set_memory_xx() consistent with x86, s390 and arm64 and makes apply_to_page_range() not to BUG() when loading modules.

Signed-off-by: Mika Penttilä <mika.penttila@nextfour.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>