blkmapd: Use getconf(_SC_PAGE_SIZE)

PAGE_SIZE is not exported by all architectures as it is not fixed: it
can depend on the model of the machine. So it's better to query the
system configuration for the actual page size on the machine.

Signed-off-by: Luk Claes <luk@debian.org>
Signed-off-by: Steve Dickson <steved@redhat.com>