Extend stripeoffset and stripesize of GEOMs from u_int to off_t

GEOM's stripeoffset overflows at 4 gigabyte margin (2^32)
because of its u_int type. This leads to incorrect data in the output
generated by "sysctl kern.geom.confxml" command, "graid list" etc.
when GEOM array has volumes larger than 4G, for example.

This change does not affect ABI but changes KBI. No MFC planned.

Differential Revision: https://reviews.freebsd.org/D13426