md: deprecate bitmap file support

The support for bitmaps on files is a very bad idea abusing various kernel
APIs, and fundamentally requires the file to not be on the actual array
without a way to check that this is actually the case.  Add a deprecation
warning to see if we might be able to eventually drop it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230615064840.629492-12-hch@lst.de

drivers/md/Kconfig

@@ -51,7 +51,7 @@ config MD_AUTODETECT
 	  If unsure, say Y.
 
 config MD_BITMAP_FILE
-	bool "MD bitmap file support"
+	bool "MD bitmap file support (deprecated)"
 	default y
 	help
 	  If you say Y here, support for write intent bitmaps in files on an

drivers/md/md.c

@@ -7049,6 +7049,8 @@ static int set_bitmap_file(struct mddev *mddev, int fd)
 				mdname(mddev));
 			return -EINVAL;
 		}
+		pr_warn("%s: using deprecated bitmap file support\n",
+			mdname(mddev));
 
 		f = fget(fd);