bcache: disassemble the big if() checks in bch_cache_set_alloc()

In bch_cache_set_alloc() there is a big if() checks combined by 11 items
together. When this big if() statement fails, it is difficult to tell
exactly which item fails indeed.

This patch disassembles this big if() checks into 11 single if() checks,
which makes code debug more easier.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>