zfs-functions.in: in_mtab() always returns 1

 zfs-functions.in: in_mtab() always returns 1

$fs used with the wrong sed command where should be $mntpnt instead
to match a variable exported by read_mtab()

The fix is mostly to reuse the sed command found in read_mtab()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Signed-off-by: Alexey Smirnoff <fling@member.fsf.org>
Closes #9168