The low-latency unmap_vmas patch silently moved the zap_bytes test after the
TLB finish and lockbreak and regather: why?  That not only makes zap_bytes
redundant (might as well use ZAP_BLOCK_SIZE), it makes the unmap_vmas level
redundant too - it's all about saving TLB flushes when unmapping a series of
small vmas.

Move zap_bytes test back before the lockbreak, and delete the curious comment
that a small zap block size doesn't matter: it's true need_flush prevents TLB
flush when no page has been unmapped, but unmapping pages in small blocks
involves many more TLB flushes than in large blocks.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>