The FS_IOC_{GET,SET}FLAGS ioctl numbers are not stable across different
architectures. Most of them use the asm-generic versions, but ALPHA and
SPARC in particular use completely different IOC number setups (see the
definition of _IOC, _IOR, _IOW, etc. in the kernel).
This commit moves the helpers for `FS_IMMUTABLE_FL` into
`osbuild/util/` and adds explicit tests. This will make sure that we
catch any ioctl mismatches as soon as possible when we run the osbuild
test-suite on other architectures. Until then, we will have to live with
this mismatch.
