fs: fix name overwrite in __register_chrdev_region()

It's possible to register a chrdev with a name size exactly the same as
was allocated in structure.  It seems it was not intended behaviour.

At least chrdev_show does not like it.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>