apparmor: fix an error code in __aa_create_ns()

apparmor: fix an error code in __aa_create_ns()

BugLink: https://bugs.launchpad.net/bugs/1835972
[ Upstream commit 0a6b29230ec336189bab32498df3f06c8a6944d8 ]

We should return error pointers in this function.  Returning NULL
results in a NULL dereference in the caller.

Fixes: 73688d1ed0b8 ("apparmor: refactor prepare_ns() and make usable from different views")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>