msdosfs: Prevent buffer overflow when expanding win95 names

In win2unixfn() we expand Windows 95 style long names. In some cases that
requires moving the data in the nbp->nb_buf buffer backwards to make room. That
code failed to check for overflows, leading to a stack overflow in win2unixfn().

We now check for this event, and mark the entire conversion as failed in that
case. This means we present the 8 character, dos style, name instead.

PR: 204643
Differential Revision: https://reviews.freebsd.org/D6015