pagecache: map zero page instead of ARC page for hole in a file

Attempt to get read ARC buffer for a hole in a file results in temporary
ARC buffer which is destroyed immediately after use. It means that
mapping such buffer is impossible, it is unmapped before page fault
handler return to application. The patch solves this by detecting that
hole in a file is accessed and mapping special zero page instead. It is
mapped as COW, so on write attempt new page is allocated.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>