Tie the name limit of a VM to SPECNAMELEN from devfs instead of a
hard-coded value. Don't allocate space for it from the kernel stack.
Account for prefix, suffix, and separator space in the name. This
takes the effective length up to 229 bytes on 13-current, and 37 bytes
on 12-stable. 37 bytes is enough to hold a full GUID string.

PR: 234134
MFC after: 1 week
Differential Revision: http://reviews.freebsd.org/D20924