bcachefs: bch2_count_fsck_err()

Factor out a helper from __bch2_fsck_err(), for counting the error in
the superblock and deciding whether to print or ratelimit - will be used
to replace some log_fsck_err() calls, where we want to lift out printing
the error message.

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