commoncap: Handle memory allocation failure.

syzbot is reporting NULL pointer dereference at xattr_getsecurity() [1],
for cap_inode_getsecurity() is returning sizeof(struct vfs_cap_data) when
memory allocation failed. Return -ENOMEM if memory allocation failed.

[1] https://syzkaller.appspot.com/bug?id=a55ba438506fe68649a5f50d2d82d56b365e0107

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 8db6c34 ("Introduce v3 namespaced file capabilities")
Reported-by: syzbot <syzbot+9369930ca44f29e60e2d@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org> # 4.14+
Acked-by: Serge E. Hallyn <serge@hallyn.com>
Acked-by: James Morris <james.morris@microsoft.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Author: Tetsuo Handa

security/commoncap.c

@@ -449,6 +449,8 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
 				magic |= VFS_CAP_FLAGS_EFFECTIVE;
 			memcpy(&cap->data, &nscap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
 			cap->magic_etc = cpu_to_le32(magic);
+		} else {
+			size = -ENOMEM;
 		}
 	}
 	kfree(tmpbuf);