ima: add fs_subtype condition for distinguishing FUSE instances

ima: add fs_subtype condition for distinguishing FUSE instances

Linux systems often use FUSE for several different purposes, where the
contents of some FUSE instances can be of more interest for auditing
than others.

Allow distinguishing between them based on the filesystem subtype
(s_subtype) using the new condition "fs_subtype".

The subtype string is supplied by userspace FUSE daemons
when a FUSE connection is initialized, so policy authors who want to
filter based on subtype need to ensure that FUSE mount operations are
sufficiently audited or restricted.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>