sched: Add unlikey branch hints to several system calls

sched: Add unlikey branch hints to several system calls

Adding an unlikely() hint on early error return paths improves the
run-time performance of several sched related system calls.

Benchmarking on an i9-12900 shows the following per system call
performance improvements:

		       before     after     improvement
sched_getattr          182.4ns    170.6ns      ~6.5%
sched_setattr          284.3ns    267.6ns      ~5.9%
sched_getparam         161.6ns    148.1ns      ~8.4%
sched_setparam        1265.4ns   1227.6ns      ~3.0%
sched_getscheduler     129.4ns    118.2ns      ~8.7%
sched_setscheduler    1237.3ns   1216.7ns      ~1.7%

Results are based on running 20 tests with turbo disabled (to reduce
clock freq turbo changes), with 10 second run per test based on the
number of system calls per second. The % standard deviation of the
measurements for the 20 tests was 0.05% to 0.40%, so the results are
reliable.

Tested on kernel build with gcc 14.2.1

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250219142423.45516-1-colin.i.king@gmail.com