UPSTREAM: fscrypto: add authorization check for setting encryption policy

commit 163ae1c6ad6299b19e22b4a35d5ab24a89791a98 upstream.

On an ext4 or f2fs filesystem with file encryption supported, a user
could set an encryption policy on any empty directory(*) to which they
had readonly access.  This is obviously problematic, since such a
directory might be owned by another user and the new encryption policy
would prevent that other user from creating files in their own directory
(for example).

Fix this by requiring inode_owner_or_capable() permission to set an
encryption policy.  This means that either the caller must own the file,
or the caller must have the capability CAP_FOWNER.

(*) Or also on any regular file, for f2fs v4.6 and later and ext4
    v4.8-rc1 and later; a separate bug fix is coming for that.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 8d693a2e67b5793ee58d106fded28902b7fd0f72)
Change-Id: I4621243c8a21a25450ee06b7e64304e9cbdfae7d
Signed-off-by: Eric Biggers <ebiggers@google.com>