pstore: selinux: add security in-core xattr support for rootfs, pstore and debugfs

- add "rootfs", "pstore" and "debugfs" to list of in-core exceptions
- change fstype checks to boolean equation
- change from strncmp to strcmp for checking

(Cherry Pick from commit 2294d499)

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 18917345
Bug: 18935184
Change-Id: Ib648f30ce4b5d6c96f11465836d6fee89bec1c72