mm/readahead.c: remove unused file_ra_state from count_history_pages

mm/readahead.c: remove unused file_ra_state from count_history_pages

count_history_pages does only call page_cache_prev_hole in rcu_lock
context using address_space mapping.  There's no need to have
file_ra_state here.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>