fix: adjust parse_size() to the unsigned size variable

fix: adjust parse_size() to the unsigned size variable

An error in parse_size() should be reported by 0, not -1,
because -1 is changed to the max value of unsigned long long
during calculations of size (e.g. at mdadm.c:412).

A negative value of size should be reported as error
(e.g. size equal -1 has been changed to the max value of
unsigned long long so far).

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>