ZTS: Update zfs_share_concurrent_shares.ksh

Occasionally an out of memory error is hit by this test case
when mounting the filesystems. Try and reduce the likelihood
of this occurring by reducing the thread count from 100 to 50.
It also has the advantage of slightly speeding up the test.

    cannot mount 'testpool/testfs3/79': Cannot allocate memory
        filesystem successfully created, but not mounted

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11283