mtd: mtdchar: Do not corrupt backing device of device node inode

We cannot modify file->f_mapping->backing_dev_info, because it will corrupt
backing device of device node inode, since file->f_mapping is equal to
inode->i_mapping (see __dentry_open() in fs/open.c).

Let's introduce separate inode for MTD device with appropriate backing
device.

[dwmw2: Refactor to keep it all entirely within mtdchar.c; use iget_locked()]

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>