bcachefs: make durability a read-write sysfs option

bcachefs: make durability a read-write sysfs option

Sometimes the user may need to change durability after formatting to
match current hardware setup, this option provides a quick and flexible
alternative to removing then adding the device.
It is HIGHLY ADVISED TO RUN REREPLICATE after changing this value so the
system doesn't remain degraded.

Signed-off-by: Daniel Hill <daniel@gluo.nz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>