psi: allow unprivileged users with CAP_SYS_RESOURCE to write psi files

Currently only root can write files under /proc/pressure. Relax this to
allow tasks running as unprivileged users with CAP_SYS_RESOURCE to be
able to write to these files.

Signed-off-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20210402025833.27599-1-johunt@akamai.com