fspacectl(2): Changes on rmsr.r_offset's minimum value returned

rmsr.r_offset now is set to rqsr.r_offset plus the number of bytes
zeroed before hitting the end-of-file. After this change rmsr.r_offset
no longer contains the EOF when the requested operation range is
completely beyond the end-of-file. Instead in such case rmsr.r_offset is
equal to rqsr.r_offset. Callers can obtain the number of bytes zeroed
by subtracting rqsr.r_offset from rmsr.r_offset.

Sponsored by: The FreeBSD Foundation
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31677