tests: cp/sparse-perf: make more robust and add zfs comments

tests: cp/sparse-perf: make more robust and add zfs comments

* init.cfg (seek_data_capable_): Add a timeout to ensure failure for
slow lseek(...SEEK_DATA) calls (even if that syscall isn't interrupted).
* tests/cp/sparse-perf.sh: Run the SEEK_DATA check on the
1TiB empty file to exclude both FreeBSD 9.1 which takes 35s,
and ZFS which requires a delay of about 5s between file creation
and use of SEEK_DATA to correctly determine it's empty (return ENXIO).
Also remove the stat size checks as they invalidate the test
due to cp never writing data due to it being always zeros,
and thus converted to holes in the output.