Do not allow the SNPSTTY ioctl for the snoop device that has a tty attached. Otherwise, the snp->snp_tty would be overwritten, while the tty line discipline still set to the snpdisc. Then snplwrite() causes panic because ttytosnp() cannot find the snp.
Do not allow the SNPSTTY ioctl for the snoop device that has a tty
attached. Otherwise, the snp->snp_tty would be overwritten, while the
tty line discipline still set to the snpdisc. Then snplwrite() causes
panic because ttytosnp() cannot find the snp.

MFC after:	1 week

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