blk/bluestore: remove BlockDevice::supported_bdev_label()

[aclamk]

Block device got this interface function with #7145.
In bluestore, bdev label just means "first disk block, located at #0".
For other devices, there is deeper meaning: https://docs.pmem.io/ndctl-user-guide/managing-label-storage-areas-lsa

It is most likely that two distinct features were joined by name.
It is obvious that any block device will be able to write to its first block.

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[ifed01]

It looks to me that at least for NVMEDevice the path BlueStore uses doesn't refer to a block device under /dev but rather some spdk config file - see NVMEDevice::open(const string& p).
Hence one shouldn't write to the first block of this "device" file and supported_bdev_label() was intended to handle that.

[aclamk]

It looks to me that at least for NVMEDevice the path BlueStore uses doesn't refer to a block device under /dev but rather some spdk config file - see NVMEDevice::open(const string& p).
Hence one shouldn't write to the first block of this "device" file and supported_bdev_label() was intended to handle that.

Right!
It means this PR depends on a9be5e2 .
We need to read bdev through bdev API, not with posix hacks.

[aclamk]

DNM until rebased on reading label through bdev