tick/sched: Consolidate idle time fetching APIs

Fetching the idle cputime is available through a variety of accessors all
over the place depending on the different accounting flavours and needs:

  - idle vtime generic accounting can be accessed by kcpustat_field(),
    kcpustat_cpu_fetch(), get_idle/iowait_time() and
    get_cpu_idle/iowait_time_us()

  - dynticks-idle accounting can only be accessed by get_idle/iowait_time()
    or get_cpu_idle/iowait_time_us()

  - CONFIG_NO_HZ_COMMON=n idle accounting can be accessed by kcpustat_field()
    kcpustat_cpu_fetch(), or get_idle/iowait_time() but not by
    get_cpu_idle/iowait_time_us()

Moreover get_idle/iowait_time() relies on get_cpu_idle/iowait_time_us()
with a non-sensical conversion to microseconds and back to nanoseconds on
the way.

Start consolidating the APIs with removing get_idle/iowait_time() and make
kcpustat_field() and kcpustat_cpu_fetch() work for all cases.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20260508131647.43868-13-frederic@kernel.org

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