lazy-pages: handle exit() based on uffdio_copy return value

The UFFDIO_EVENT_EXIT didn't make it upstream because of possible races in
exit() syscall [1].
The only way to detect that the monitored process is exited is checking for
ENOSPC errno value set by uffdio_copy.

[1] http://www.spinics.net/lists/linux-mm/msg122467.html



Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>