xfs: move kmem_to_page()

Move it to the general xfs linux wrapper header file so we can
prepare to remove kmem.h

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>