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 inc_nlink(struct inode *inode)
{
	if (unlikely(inode->i_nlink == 0)) {
		WARN_ON(!(inode->i_state & I_LINKABLE));
		atomic_long_dec(&inode->i_sb->s_remove_count);
	}

	inode->__i_nlink++;
}

void inc_nlink(struct inode *inode)
{
	if (unlikely(inode->i_nlink == 0)) {
		WARN_ON(!(inode->i_state & I_LINKABLE));
		atomic_long_dec(&inode->i_sb->s_remove_count);
	}

	inode->__i_nlink++;
}
