removed extra extern file_fdatawait_range

Jeff added this extern twice in commit a823e458

Fixes: a823e458

 ("mm: add file_fdatawait_range and file_write_and_wait")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>