lib/deploy: Ignore FIFREEZE/FITHAW errors when already in state

lib/deploy: Ignore FIFREEZE/FITHAW errors when already in state

If the filesystem is already frozen, FIFREEZE returns EBUSY, and if the
filesystem is already thawed, FITHAW returns EINVAL. It's very unlikely
these issues would arise on a real ostree system since the sysroot would
be locked during the freeze/thaw cycle.

However, when multiple fake sysroots are used during the test suite (run
as root), the tests could race to run the freeze/thaw cycle without
locking. Furthermore, there's no reason why an independent process might
be trying to freeze the filesystem while ostree was deploying. Ignore
but warn for these errors since there's not much ostree can do about it,
anyways.

Closes: #1260
Approved by: cgwalters