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 inode_set_flags(struct inode *inode, unsigned int flags,
		     unsigned int mask)
{
	unsigned int old_flags, new_flags;

	WARN_ON_ONCE(flags & ~mask);
	do {
		old_flags = ACCESS_ONCE(inode->i_flags);
		new_flags = (old_flags & ~mask) | flags;
	} while (unlikely(cmpxchg(&inode->i_flags, old_flags,
				  new_flags) != old_flags));
}

void inode_set_flags(struct inode *inode, unsigned int flags,
		     unsigned int mask)
{
	unsigned int old_flags, new_flags;

	WARN_ON_ONCE(flags & ~mask);
	do {
		old_flags = ACCESS_ONCE(inode->i_flags);
		new_flags = (old_flags & ~mask) | flags;
	} while (unlikely(cmpxchg(&inode->i_flags, old_flags,
				  new_flags) != old_flags));
}
