test_fsync file overrun

test_fsync in tools/fsync pre-creates a 16MB file. If it is given a number
of iterations greater than 1024 (like one might use if trying to see what
happens when NVRAM gets filled, or on a journaling file system), than one of
the writes being timed will have to extend the size of the pre-created test
file, which can greatly skew the results.

This patch uses lseek to periodically restart at the beginning of the file,
rather than writing past the end of it.

Cheers,

Jeff