ufs proper handling of zero link case

This patch should fix or partly fix this bug:
http://bugzilla.kernel.org/show_bug.cgi?id=8276



The problem is:

- if we see "zero link case" during reading inode operation, we call
  ufs_error(which remount fs readonly), but not "mark" inode as bad (1)

- in readonly case we do not fill some data structures, which are used in
  read and write case (2)

- VFS call ufs_delete_inode if link count is zero (3)

so (1)->(3)->(2) cause oops, this patch should fix such scenario

Signed-off-by: Evgeniy Dushistov <dushistov@mail.ru>
Cc: Jim Paris <jim@jtan.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>