vfs: count unlinked inodes

vfs: count unlinked inodes

Add a new counter to the superblock that keeps track of unlinked but
not yet deleted inodes.

Do not WARN_ON if set_nlink is called with zero count, just do a
ratelimited printk.  This happens on xfs and probably other
filesystems after an unclean shutdown when the filesystem reads inodes
which already have zero i_nlink.  Reported by Christoph Hellwig.

Signed-off-by: Miklos Szeredi <[email protected]>
Tested-by: Toshiyuki Okajima <[email protected]>
Signed-off-by: Al Viro <[email protected]>