tools/perf: in builtin-kvm, initialize file_name

Currently, if no options are passed to `perf kvm report`, the file_name
is uninitialized, leading to bad behavior when strdup() is called on it.

Initialize to NULL to correct.
dnuma.h fixup
memlayout.h updates
dnuma.c fixups.
dnuma.c debugfs fixups.
memlayout.c fixups.
memlayout.c debugfs fixups.