x86, bitops: Move fls64.h inside __KERNEL__

We would include <asm-generic/bitops/fls64.h> even without __KERNEL__,
but that doesn't make sense, as:

1. That file provides fls64(), but the corresponding function fls() is
   not exported to user space.
2. The implementation of fls64.h uses kernel-only symbols.
3. fls64.h is not exported to user space.

This appears to have been a bug introduced in checkin:

d57594c2 bitops: use __fls for fls64 on 64-bit archs

Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Alexander van Heukelum <heukelum@mailshack.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/4EEA77E1.6050009@zytor.com