x86/mm: Tidy up "x86/mm: Flush more aggressively in lazy TLB mode"

x86/mm: Tidy up "x86/mm: Flush more aggressively in lazy TLB mode"

Due to timezones, commit:

  b956575bed91 ("x86/mm: Flush more aggressively in lazy TLB mode")

was an outdated patch that well tested and fixed the bug but didn't
address Borislav's review comments.

Tidy it up:

 - The name "tlb_use_lazy_mode()" was highly confusing.  Change it to
   "tlb_defer_switch_to_init_mm()", which describes what it actually
   means.

 - Move the static_branch crap into a helper.

 - Improve comments.

Actually removing the debugfs option is in the next patch.

Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: b956575bed91 ("x86/mm: Flush more aggressively in lazy TLB mode")
Link: http://lkml.kernel.org/r/154ef95428d4592596b6e98b0af1d2747d6cfbf8.1508000261.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>