powerpc: Implement __get_user_pages_fast()

Other architectures have a __get_user_pages_fast(), in addition to the
regular get_user_pages_fast(), which doesn't call get_user_pages() on
failure, and thus doesn't attempt to fault pages in or COW them.  The
generic KVM code uses __get_user_pages_fast() to detect whether a page
for which we have only requested read access is actually writable.

This provides an implementation of __get_user_pages_fast() by
splitting the existing get_user_pages_fast() in two.  With this, the
generic KVM code will get the right answer instead of always
considering such pages non-writable.

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