Fix segfault in zfs_do_bookmark()

When invoked with wrong parameters 'zfs bookmark' fails to gracefully
validate user input and crashes.

This is a regression accidentally introduced in 587e228; this commit
adds additional tests to the ZFS Test Suite to exercise this codepath.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: KireinaHoro <i@jsteward.moe>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7228
Closes #7229