bcachefs: BCH_SB_VERSION_INCOMPAT

bcachefs: BCH_SB_VERSION_INCOMPAT

We've been getting away from feature bits: they don't have any kind of
ordering, and thus it's possible for people to enable weird combinations
of features that were never tested or intended to be run.

Much better to just give every new feature, compatible or incompatible,
a version number.

Additionally, we probably won't ever rev the major version number: major
version numbers represent incompatible versions, but that doesn't really
fit with how we actually roll out incompatible features - we need a
better way of rolling out incompatible features.

So, this patch adds two new superblock fields:
- BCH_SB_VERSION_INCOMPAT
- BCH_SB_VERSION_INCOMPAT_ALLOWED

BCH_SB_VERSION_INCOMPAT_ALLOWED indicates that incompatible features up
to version number x are allowed to be used without user prompting, but
it does not by itself deny old versions from mounting.

BCH_SB_VERSION_INCOMPAT does deny old versions from mounting, and must
be <= BCH_SB_VERSION_INCOMPAT_ALLOWED.

BCH_SB_VERSION_INCOMPAT will only be set when a codepath attempts to use
an incompatible feature, so as to not unnecessarily break compatibility
with old versions.

bch2_request_incompat_feature() is the new interface to check if an
incompatible feature may be used.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>