The freebsd4_getfsstat() was broken in r281551 to always return 0 on success.
All versions of getfsstat(3) are supposed to return the number of [o]statfs
structs in the array that was copied out.

Also fix missing bounds checking and signed comparison of unsigned types.

Submitted by: bde@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation