sh: Correct iounmap fixmap teardown.

iounmap_fixed() had a couple of bugs in it that caused it to effectively
fail at life. The total number of pages to unmap factored in the mapping
offset and aligned up to the next page boundary, which doesn't match the
ioremap_fixed() behaviour.

When ioremap_fixed() pegs a slot, the address in the mapping data already
contains the offset displacement, and the size is recorded verbatim given
that we're only interested in total number of pages required. As such, we
need to calculate the total number from the original size in the unmap
path as well.

At the same time, there was also an off-by-1 problem in the fixmap index
calculation which has also been corrected.

Previously subsequent remaps of an identical fixmap index would trigger
the pte_ERROR() in set_pte_phys():

	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).
	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).
	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).
	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).
	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).
	arch/sh/mm/init.c:77: bad pte 8053ffb0(0000781003fff506).

With this patch in place, the iounmap-driven fixmap teardown actually
does what it's supposed to do.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>