block: avoid buffer overrun by using pstrcpy, not strncpy

Also, use PATH_MAX, rather than the arbitrary 1024.
Using PATH_MAX is more consistent with other filename-related
variables in this file, like backing_filename and tmp_filename.

Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
