mount: Handle EROFS before calling mount() syscall

mount: Handle EROFS before calling mount() syscall

If the loop device is already initialized read-only, the new code for loop
device reuse returns -EROFS. There is no solution of this situation. But mount
can behave in the same way, as it does for EROFS returned by mount syscall: Try
again in read-only mode.

Before:
mount: /mnt/2: mount failed: Read-only file system

After:
mount: /btrfs.img is used as read only loop, mounting read-only

Note: It would be nice to mention loop device name in the warning message, but
it is not available in the mount context.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>