bcachefs: Don't set BCH_FEATURE_incompat_version_field unless requested

We shouldn't be setting incompatible bits or the incompatible version
field unless explicitly request or allowed - otherwise we break mounting
with old kernels or userspace.

Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>