x86, fpu: drop the fpu state during thread exit

There is no need to save any active fpu state to the task structure
memory if the task is dead. Just drop the state instead.

For example, this saved some 1770 xsave's during the system boot
of a two socket Xeon system.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1336692811-30576-4-git-send-email-suresh.b.siddha@intel.com
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>