affs_do_readpage_ofs(): just use kmap_atomic() around memcpy()

It forgets kunmap() on a failure exit, but there's really no point keeping
the page kmapped at all - after all, what we are doing is a bunch of memcpy()
into the parts of page, so kmap_atomic()/kunmap_atomic() just around those
memcpy() is enough.

Spotted-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>