x86/fpu: Document user_fpu_begin()

x86/fpu: Document user_fpu_begin()

Currently, user_fpu_begin() has a single caller and it is not clear why
do we actually need it and why we should not worry about preemption
right after preempt_enable().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pekka Riikonen <priikone@iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150311173409.GC5032@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>