The problem is, if we increase our readhead size arbitrarily (say 2M), we
call mpage_readpages() with 2M and when it tries to allocated a bio enough to
fit 2M it fails, then we kick it back to "confused" code - which does 4K at
a time.

The fix is to ask for the maxium the driver can handle.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>