get rid of perf_fget_light(), convert kernel/events/core.c to CLASS(fd)

Lift fdget() and fdput() out of perf_fget_light(), turning it into
is_perf_file(struct fd f).  The life gets easier in both callers
if we do fdget() unconditionally, including the case when we are
given -1 instead of a descriptor - that avoids a reassignment in
perf_event_open(2) and it avoids a nasty temptation in _perf_ioctl()
where we must *not* lift output_event out of scope for output.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>