Don't count '@' for dataset namelen if not a snapshot

Don't count '@' for dataset namelen if not a snapshot

Don't count '@' for dataset namelen if not a snapshot.  This
fixes making a pool unimportable when the  dataset namelen
is 255.

Add test file for zfs create name length 255.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #5432
Closes #5456