pre-dump: handle errors from page_xfer_dump_pages()

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
deduplication: make seek_pagemap_page properly handle eof

because eof in restore means that we don't have "must have" data, so
it mean error exit, but in dedup, it's ok not to have data we seek, so
we continue returning normally

also it's used that if off < pr-cvaddr then no eof happened and if eof
happened then off >= pr-cvaddr(then return of seek_pagemap_page is 0)

Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>