xfs_io: don't walk off the end of argv in fzero_f

xfs_io: don't walk off the end of argv in fzero_f

The fzero_f function doesn't check that there are enough non-switch
parameters to supply offset and length arguments to fallocate.  As a
result, we can walk off the end of the argv array and crash.  A
secondary problem is that we don't use getopt to detect the -k, which is
not how most xfs_io commands work.

Therefore, use getopt to detect the -k argument and rewire the offset
and length interpretation code to check optind and use argv correctly.
This bug is trivially reproduced by "xfs_io -c 'fzero -k 0' /some/file".

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>