x86/apic: Deinline _flat_send_IPI_mask, save ~150 bytes

_flat_send_IPI_mask: 157 bytes, 3 callsites

     text     data      bss       dec     hex filename
 96183823 20860520 36122624 153166967 9212477 vmlinux1_before
 96183699 20860520 36122624 153166843 92123fb vmlinux

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Borislav Petkov <bp@alien.de>
Cc: Daniel J Blueman <daniel@numascale.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Travis <travis@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1457287876-6001-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>