bcachefs: add no_invalid_checks flag

Setting this flag on a filesystem results in validity checks being
skipped when writing bkeys. This flag will be used by tooling that
deliberately injects corruption into a filesystem in order to exercise
fsck. It shouldn't be set outside of testing/debugging code.

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>