asm-generic/tlb: stub out pmd_free_tlb() if nopmd

asm-generic/tlb: stub out pmd_free_tlb() if nopmd

This came up when removing __ARCH_HAS_5LEVEL_HACK for ARC as code bloat.
With this patch we see the following code reduction.

| bloat-o-meter2 vmlinux-E-elide-p?d_clear_bad vmlinux-F-elide-pmd_free_tlb
| add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-112 (-112)
| function                                     old     new   delta
| free_pgd_range                               422     310    -112
| Total: Before=4137042, After=4136930, chg -1.000000%

Note that pmd folding can be tricky: In 2-level setup (where pmd is
conceptually folded) most pmd routines are valid and refer to upper levels.
In this patch we can, but see next patch for example where we can't

Link: http://lkml.kernel.org/r/20191016162400.14796-5-vgupta@synopsys.com
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>