floppy: bail out in open() if drive is not responding to block0 read

In case reading of block 0 during open() fails, it is not the right thing
to let open() succeed.

Fix this by introducing FD_OPEN_SHOULD_FAIL_BIT flag, and setting it in
case the bio callback encounters an error while trying to read block 0.

As a bonus, this works around certain broken userspace (blkid), which is
not able to properly handle read()s returning IO errors. Hence be nice to
those, and bail out during open() already; if block 0 is not readable,
read()s are not going to provide any meaningful data anyway.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/uapi/linux/fd.h

@@ -185,7 +185,8 @@ enum {
 				 * to clear media change status */
 	FD_UNUSED_BIT,
 	FD_DISK_CHANGED_BIT,	/* disk has been changed since last i/o */
-	FD_DISK_WRITABLE_BIT	/* disk is writable */
+	FD_DISK_WRITABLE_BIT,	/* disk is writable */
+	FD_OPEN_SHOULD_FAIL_BIT
 };
 
 #define FDSETDRVPRM _IOW(2, 0x90, struct floppy_drive_params)