savecore: decrease filename buffer sizes

All files are now created relative to savedirfd, e.g. with openat(2).
Therefore, we do not need character buffers to be PATH_MAX bytes long,
just long enough to hold the complete filename. 32 bytes is long enough
in all cases. These can be allocated on the stack.

While here, fix an error message that attempts to use an uninitialized
infoname.

Reviewed by: markj
MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34821