sh: Fix mmap2 for handling differing PAGE_SIZEs.

mmap2 uses a fixed page shift of 12, regardless of the PAGE_SIZE setting.
Fix up the mmap2 code to add some sanity checks on the mapping, and to
update pgoff accordingly.

Error handling bits based on 4280e312


("frv: fix mmap2 error handling").

Signed-off-by: Toshinobu Sugioka <sugioka@itonet.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>