bcachefs: version_upgrade is now an enum

bcachefs: version_upgrade is now an enum

The version_upgrade parameter is now an enum, not a bool, and it's
persistent in the superblock:
 - compatible (default):	upgrade to the latest compatible version
 - incompatible:		upgrade to latest incompatible version
 - none

Currently all upgrades are incompatible upgrades, but the next release
will introduce major:minor versions.

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