readahead: return early when readahead is disabled

Reduce readahead overheads by returning early in do_sync_mmap_readahead().

tmpfs has ra_pages=0 and it can page fault really fast (not constraint by
IO if not swapping).

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Tim Chen <tim.c.chen@intel.com>
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>