x86/fpu: Fix invalid FPU ptrace state after execve()

x86/fpu: Fix invalid FPU ptrace state after execve()

Robert O'Callahan reported that after an execve PTRACE_GETREGSET
NT_X86_XSTATE continues to return the pre-exec register values
until the exec'ed task modifies FPU state.

The test code is at:

  https://bugzilla.redhat.com/attachment.cgi?id=1164286.

What is happening is fpu__clear() does not properly clear fpstate.
Fix it by doing just that.

Reported-by: Robert O'Callahan <[email protected]>
Signed-off-by: Yu-cheng Yu <[email protected]>
Cc: <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Hansen <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>