block: stub out and deprecated the capability attribute on the gendisk

The capability attribute was added in 2017 to expose the kernel internal
GENHD_FL_MEDIA_CHANGE_NOTIFY to userspace without ever adding a value to
an UAPI header, and without ever setting it in any driver until it was
finally removed in Linux 5.7.

Deprecate the file and always return 0 instead of exposing the other
internal and frequently renumbered other gendisk flags.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230203150209.3199115-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/capability.rst

-===============================
-Generic Block Device Capability
-===============================
-
-This file documents the sysfs file ``block/<disk>/capability``.
-
-``capability`` is a bitfield, printed in hexadecimal, indicating which
-capabilities a specific block device supports:
-
-.. kernel-doc:: include/linux/blkdev.h

Documentation/block/index.rst

@@ -10,7 +10,6 @@ Block
    bfq-iosched
    biovecs
    blk-mq
-   capability
    cmdline-partition
    data-integrity
    deadline-iosched

block/genhd.c

@@ -1024,9 +1024,8 @@ ssize_t part_inflight_show(struct device *dev, struct device_attribute *attr,
 static ssize_t disk_capability_show(struct device *dev,
 				    struct device_attribute *attr, char *buf)
 {
-	struct gendisk *disk = dev_to_disk(dev);
-
-	return sprintf(buf, "%x\n", disk->flags);
+	dev_warn_once(dev, "the capability attribute has been deprecated.\n");
+	return sprintf(buf, "0\n");
 }
 
 static ssize_t disk_alignment_offset_show(struct device *dev,