r5cache: set MD_JOURNAL_CLEAN correctly

Currently, the code sets MD_JOURNAL_CLEAN when the array has
MD_FEATURE_JOURNAL and the recovery_cp is MaxSector. The array
will be MD_JOURNAL_CLEAN even if the journal device is missing.

With this patch, the MD_JOURNAL_CLEAN is only set when the journal
device presents.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>