ANDROID: overlayfs: add __get xattr method

Because of the overlayfs getxattr recursion, the incoming inode fails
to update the selinux sid resulting in avc denials being reported
against a target context of u:object_r:unlabeled:s0.

Solution is to add a _get xattr method that calls the __vfs_getxattr
handler so that the context can be read in, rather than being denied
with an -EACCES when vfs_getxattr handler is called.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 133515582
Bug: 136124883
Bug: 129319403
Change-Id: Ia39543c5ce617976f14d790fb88e471d575ffd65