For requests marked read-ahead, it can legitimately fail without it being a
path problem.  So don't fail a path just because this happens (can be the
atomic request allocation going nuts, for instance), or all paths will
quickly go away.

Cc: <neilb@cse.unsw.edu.au>
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>