Fix a stack overflow in mount_smbfs when hostname is too long.

Fix a stack overflow in mount_smbfs when hostname is too long.

The local hostname was blindly copied into the to the nn_name array.
When the hostname exceeded 16 bytes, it would overflow.  Truncate the
hostname to 15 bytes plus a 0 terminator which is the "workstation name"
suffix.

Use defensive strlcpy() when filling nn_name in all cases.

PR:		228354
Reported by:	donald.buchholz@intel.com
Reviewed by:	jpaetzel,  ian (prior version)
Discussed with:	Security Officer (gtetlow)
MFC after:	3 days
Security:	Stack overflow with the hostname.
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D15936