sched/core: Silence a warning in sched_init()

Compiling a kernel with both FAIR_GROUP_SCHED=n and RT_GROUP_SCHED=n
will generate a compiler warning:

  kernel/sched/core.c: In function 'sched_init':
  kernel/sched/core.c:5906:32: warning: variable 'ptr' set but not used

It is unnecessary to have both "alloc_size" and "ptr", so just combine
them.

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: valentin.schneider@arm.com
Link: https://lkml.kernel.org/r/20190720012319.884-1-cai@lca.pw
Signed-off-by: Ingo Molnar <mingo@kernel.org>