perf tools: Use snprintf() for root_dir path construction

perf tools: Use snprintf() for root_dir path construction

get_kernel_version() in machine.c and dso__load_guest_kernel_sym() in
symbol.c use sprintf() to construct paths by prepending root_dir to
"/proc/version" and "/proc/kallsyms" respectively.  Both write into
PATH_MAX stack buffers, but root_dir comes from --guestmount or KVM
configuration and is not length-checked.  A root_dir at or near
PATH_MAX causes a stack buffer overflow.

Switch to snprintf() with sizeof(path) to prevent overflow.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: a1645ce12adb6c9c ("perf: 'perf kvm' tool for monitoring guest performance from host")
Cc: Zhang Yanmin <yanmin_zhang@linux.intel.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>