bcachefs: Fix fsck warnings from bkey validation

__bch2_fsck_err() warns if the current task has a btree_trans object and
it wasn't passed in, because if it has to prompt for user input it has
to be able to unlock it.

But plumbing the btree_trans through bkey_validate(), as well as
transaction restarts, is problematic - so instead make bkey fsck errors
FSCK_AUTOFIX, which doesn't need to warn.

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