Bug-fix in size calculation

Fixes a bug in how PARASITE_MAX_GROUPS was calculated, and adds a
compiler check to assert that parasite_dump_creds doesn't exceed
the page size.

Signed-off-by: Sophie Blee-Goldman <ableegoldman@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: bers -- Add short help output

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>