MFstable/10 r292395:

MFC r292048:

Don't leak rsector/sector in mp_label(..)

Use calloc instead of malloc + memset(.., 0, ..) when allocating sector

Differential Revision: https://reviews.freebsd.org/D4450
Reported by: cppcheck
Reviewed by: mav
Sponsored by: EMC / Isilon Storage Division