core: add RestrictFileSystemAccess= BPF LSM for dm-verity execution enforcement

Add a new RestrictFileSystemAccess= boolean setting in the [Manager] section of
system.conf that enforces execution only from signed dm-verity block
devices and the initramfs during early boot.

When RestrictFileSystemAccess=yes is set, PID1 loads a BPF LSM program early in boot
that:

Integrity tracking (self-populating, no userspace involvement):
 - bdev_setintegrity: records dm-verity signature status in a BPF hash
   map when the kernel signals device integrity via
   security_bdev_setintegrity()
 - bdev_free_security: removes devices from the map on teardown

Execution enforcement (deny-default policy):
 - bprm_check_security: blocks execve() from untrusted sources
 - mmap_file: blocks PROT_EXEC mmap (shared libs, anonymous exec memory)
 - file_mprotect: blocks W->X transitions (JIT, libffi, etc.)

Trust anchors:
 - Signed dm-verity volumes (sig_valid flag in the BPF map)
 - Initramfs (s_dev captured at load time, cleared after switch_root)
 - Everything else is denied (tmpfs, procfs, sysfs, anonymous PROT_EXEC)

PID1 requires dm-verity require_signatures=1 to be enabled and refuses
to load the BPF program otherwise, ensuring the kernel enforces that all
dm-verity devices carry valid signatures.

After attach, PID1 extracts owned FDs from the skeleton (link FDs +
.bss map FD) and lets the skeleton be destroyed. The dup'd link FDs
keep programs attached via the kernel reference chain (link FD ->
bpf_link -> bpf_prog -> bpf_map). Destroying the skeleton unmaps the
.bss page from PID1's address space so no BPF state is readable via
/proc/1/mem. The .bss map FD is retained for targeted writes (clearing
initramfs_s_dev after switch_root via mmap).

Signed-off-by: Christian Brauner <brauner@kernel.org>

man/kernel-command-line.xml

@@ -79,6 +79,17 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>systemd.restrict_filesystem_access=</varname></term>
+        <listitem>
+          <para>Controls the <varname>RestrictFileSystemAccess=</varname> execution enforcement policy. For
+          details, see
+          <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+          <xi:include href="version-info.xml" xpointer="v261"/>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>systemd.mask=</varname></term>
         <term><varname>systemd.wants=</varname></term>