fs,userns: Change inode_capable to capable_wrt_inode_uidgid

The kernel has no concept of capabilities with respect to inodes; inodes
exist independently of namespaces.  For example, inode_capable(inode,
CAP_LINUX_IMMUTABLE) would be nonsense.

This patch changes inode_capable to check for uid and gid mappings and
renames it to capable_wrt_inode_uidgid, which should make it more
obvious what it does.

Fixes CVE-2014-4014.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
void __insert_inode_hash(struct inode *inode, unsigned long hashval)
{
	struct hlist_head *b = inode_hashtable + hash(inode->i_sb, hashval);

	spin_lock(&inode_hash_lock);
	spin_lock(&inode->i_lock);
	hlist_add_head(&inode->i_hash, b);
	spin_unlock(&inode->i_lock);
	spin_unlock(&inode_hash_lock);
}

void __insert_inode_hash(struct inode *inode, unsigned long hashval)
{
	struct hlist_head *b = inode_hashtable + hash(inode->i_sb, hashval);

	spin_lock(&inode_hash_lock);
	spin_lock(&inode->i_lock);
	hlist_add_head(&inode->i_hash, b);
	spin_unlock(&inode->i_lock);
	spin_unlock(&inode_hash_lock);
}
