ZTS: Fix zfs_diff_timestamp

When using mawk instead of gawk zfs_diff_timestamp fails consistently:
this is due to a subtle difference in how mawk handles substr().

From awk(1):

Finally, here is how mawk handles exceptional cases not discussed in
the AWK book or the Posix draft. It is unsafe to assume consistency
across awks and safe to skip to the next section.
substr(s, i, n) returns the characters of s in the intersection of
the closed interval [1, length(s)] and the half-open interval [i, i+n).
When this intersection is empty, the empty string is returned; so

substr("ABC", 1, 0) = "" and substr("ABC", -4, 6) = "A".

To support running zfs_diff_timestamp with both gawk and mawk change
the second parameter passed to substr().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7503
Closes #7510