sched/psi: Fix OOB write when writing 0 bytes to PSI files

Issuing write() with count parameter set to 0 on any file under
/proc/pressure/ will cause an OOB write because of the access to
buf[buf_size-1] when NUL-termination is performed. Fix this by checking
for buf_size to be non-zero.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20200203212216.7076-1-surenb@google.com
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 6fcca0fa48118e6d63733eb4644c6cd880c15b8f
Change-Id: I733341988a26bc02be9b7013c297b0ac461eda7e
Signed-off-by: Alam Md Danish <amddan@codeaurora.org>