bcachefs: Fix BCH_FS_ERROR flag handling

We were setting BCH_FS_ERROR on startup if the superblock was marked as
containing errors, which is not what we wanted - BCH_FS_ERROR indicates
whether errors have been found, so that after a successful fsck we're
able to clear the error bit in the superblock.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>