s390/cpum_cf: Fix uninitialized warning after backport of ce971233

Upstream commit ce971233 ("s390/cpum_cf: Deny all sampling events by
counter PMU"), backported to 6.6 as commit d660c8d8 ("s390/cpum_cf:
Deny all sampling events by counter PMU"), implicitly depends on the
unconditional initialization of err to -ENOENT added by upstream
commit aa1ac982 ("s390/cpumf: Fix double free on error in
cpumf_pmu_event_init()"). The latter change is missing from 6.6,
resulting in an instance of -Wuninitialized, which is fairly obvious
from looking at the actual diff.

  arch/s390/kernel/perf_cpum_cf.c:858:10: warning: variable 'err' is uninitialized when used here [-Wuninitialized]
    858 |                 return err;
        |                        ^~~

Commit aa1ac982 ("s390/cpumf: Fix double free on error in
cpumf_pmu_event_init()") depends on commit c70ca298 ("perf/core:
Simplify the perf_event_alloc() error path"), which is a part of a much
larger series unsuitable for stable.

Extract the unconditional initialization of err to -ENOENT from
commit aa1ac982 ("s390/cpumf: Fix double free on error in
cpumf_pmu_event_init()") and apply it to 6.6 as a standalone change to
resolve the warning.

Fixes: d660c8d8 ("s390/cpum_cf: Deny all sampling events by counter PMU")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>