fgraph: Make pid_str size match the comment

The comment above buffer mentions sign, 10 bytes width for number and null
terminator, but buffer itself isn't large enough to hold that much data.

This is a cosmetic change, since PID cannot be negative, other than -1.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Link: https://lore.kernel.org/20250617152110.2530-1-a.sadovnikov@ispras.ru


Signed-off-by: Artem Sadovnikov <a.sadovnikov@ispras.ru>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>