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

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

The 'while read line; ...; done' loop is run in a piped subshell
therefore the 'return 0' would not cause a return from the
is_mounted() function.  In all cases, this function will
always return 1.

The fix is to 'return 1' from the subshell on a successful match
(no match == return 0), and then negating the final return value.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: TerraTech <TerraTech@users.noreply.github.com>
Closes #8151