core: pin restrict-fsaccess initramfs_s_dev store width to skeleton field

core: pin restrict-fsaccess initramfs_s_dev store width to skeleton field

The clear-store in restrict_fsaccess_clear_initramfs_trust() writes a fixed
4 bytes (*(uint32_t *)(p + INITRAMFS_S_DEV_OFF) = 0). INITRAMFS_S_DEV_OFF is
derived from the skeleton, so the offset tracks any field widening, but the
store width does not: were initramfs_s_dev widened (e.g. __u32 -> __u64) in
the BPF program, the store would clear only the low 4 bytes and silently
leave the initramfs trust window partially open. That is exactly the class
of bug the mirror-struct asserts (removed earlier in this branch) guarded
against.

Add a compile-time assert pinning the store width to the skeleton field
width (sizeof_field(typeof_field(struct restrict_fsaccess_bpf, bss[0]),
initramfs_s_dev) == sizeof(uint32_t)), so widening the field fails the build
instead of clearing half of it.