shared/005,7: make sure debugfs sets negative i_size

shared/005 and shared/007 work abnornamlly on RHEL6.8GA and
RHEL6.9Beta because debugfs fails to set i_size to -1 or -512
and reports "exceeds field size maximum".  When debugfs fails
to set a negative i_size, we can skip these cases which don't
trigger the kernel bug.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>