bcache: remove bcache device self-defined readahead

For read cache missing, bcache defines a readahead size for the read I/O
request to the backing device for the missing data. This readahead size
is initialized to 0, and almost no one uses it to avoid unnecessary read
amplifying onto backing device and write amplifying onto cache device.
Considering upper layer file system code has readahead logic allready
and works fine with readahead_cache_policy sysfile interface, we don't
have to keep bcache self-defined readahead anymore.

This patch removes the bcache self-defined readahead for cache missing
request for backing device, and the readahead sysfs file interfaces are
removed as well.

This is the preparation for next patch to fix potential kernel panic due
to oversized request in a simpler method.

Reported-by: Alexander Ullrich <[email protected]>
Reported-by: Diego Ercolani <[email protected]>
Reported-by: Jan Szubiak <[email protected]>
Reported-by: Marco Rebhan <[email protected]>
Reported-by: Matthias Ferdinand <[email protected]>
Reported-by: Victor Westerhuis <[email protected]>
Reported-by: Vojtech Pavlik <[email protected]>
Reported-and-tested-by: Rolf Fokkens <[email protected]>
Reported-and-tested-by: Thorsten Knabe <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: [email protected]
Cc: Kent Overstreet <[email protected]>
Cc: Nix <[email protected]>
Cc: Takashi Iwai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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